Sunday, March 22, 2020

Post #200 in the Blog of Former MySQL Entomologist!

I noted that since August 2012 when I started this blog (while planning to leave my job as a MySQL Entomologist in Oracle to see some real MySQL life besides bugs) I've already published 199 posts. In this post #200 I'd like to look back and pick up one post per each year of blogging so far that I consider really good, important or useful.

I try my best to pick up posts that are mostly not about specific MySQL bugs. These posts either caused some interesting discussions or defined my interests for a log time, or just opened new directions in my public blogging efforts. I think many of them also got less public attention than they deserve (so most popular posts are not in this list). I also try to summarize my work related memories and sentiments of the year.

Here is the list:
  • 2012. "New Year Wishes for Customers of Oracle's MySQL Technical Support Services".

    That year I considered my job roles in Oracle too boring and decided to quit and join Percona for real fun, practical experience and really good money! I also created few social media accounts and this blog, as I decided to finally share the details of my past and current work in public. I just started and published only 4 posts during that year and the one I picked up was probably created during my shift on December 31 (I still like to work on weekends and public holidays, but do this less often). Basically I wanted to encourage Oracle customers (and employees) to report MySQL bugs they found in public MySQL bugs database. Expected for the MySQL Entomologist to wish this, isn't it? I still wish the same today, even more! Unfortunately I see public MySQL bugs database is used less these days and this makes me sad.

  • 2013. "How to Drop the Trigger".

    I had a lot of work, bugs and problems to deal with in 2013, including hard to forget snow storm in March. This year I first visited UK to speak about Performance Schema at Percona Live (it was probably my worst accepted talk ever) and enjoy London and Brighton. Who could imagine the outcomes of that very first visit... I had written a lot in public that year, 42 blog posts and so many rumblings about bugs at Facebook that I annoyed many of my former colleagues. The blog post I picked up was probably my first ever "HowTo" style post in this blog and that's why it is important. It described the real life case when due to missing .TRG file DROP TRIGGER statement just failed. One had to create the file manually to proceed, and it's easy as it's a plain text file. Surely I listed some bugs I checked in the process, including those that could explain why the file was missing.

  • 2014. "On responsible bugs reporting".

    I was less active blogger in 2014, as I was kindly asked by both Oracle and Percona officials and colleagues to reduce my extraordinary bugs-related social media activity. This made me sad, but allowed to concentrate better on support and other job duties in Percona. I kept myself mostly silent after the blog post mentioned above, where I discussed what a responsible engineer should do when she finds some simple sequence of SQL statements that, when executed by authenticated user explicitly having all the privileges needed to execute these statements, crashes some version of her favorite MySQL fork. I was (and still is) against hiding this kind of crashes by Oracle or any other open source software vendor, and suggested to report them in public as I always did. Many MySQL Community members disagree with this approach even today, probably. In 2014 I've got my (first and only) talk (about MySQL bugs reporting) accepted to Oracle Open World 2014 somehow. I keep submitting multiple talks for this event every year, to no result since that time...

  • 2015. "Using gdb to understand what locks (and when) are really set by InnoDB. Part I.".

    In 2015 I started to study MySQL internals with gdb on a regular basis. The blog post I picked up for this year was the first in a series of such studies that continue until today. Initially it was as simple as setting few breakpoints and checking several lines of source code here and there, but I had not stopped at that stage. As a result of that studies, in 2015 I've presented interesting and really well accepted talks at FOSDEM and another one about InnoDB locks and deadlocks at Percona Live. Who could imagine that next time I speak at any Percona conference it will be 2019! The rest of the year was hard and sad at work, where I spent most of my time fighting for my Support team values and colleagues against the upcoming changes, new leaders and new company approaches to services. Not so many blog posts as a result, just 12.

  • 2016. "I'm Winston Wolf, I solve problems.".

    In January, 2016, my internal fights in Percona influenced content and highlights of my blog posts. One of them, my all times favorite, was aimed at explaining how Support should work and why, based on real life story with one of customer issues I resolved instantly working the way I prefer! I have to note that my feature request (Bug #76030 - "Add a way to disable rowid-ordered retrieval to optimizer_switch") that led me to the idea about the root cause is still "Open" today. Even more interesting that the issue was about MariaDB. Later I lost in all my fights for Support in Percona (fights, but not the battle, as Percona today mostly operates the way I defended back then, not the way they planned or tried to enforce) and had to quit, to end up working in MariaDB Corporation where I stay till today, and happy! In general, 2016 was my most successful year as a blogger, with up to 1000 page views per day during some months. Many of 29 blog posts published that year are worth re-reading and quoting, and became really popular, but I'd like to remind mostly about this series about MySQL Support people... Since 2016 I am less active at conferences due to my (now resolved) conflict with Percona on this topic as well, so FOSDEM was my only MySQL-related public appearance in 2016.

  • 2017. "perf Basics for MySQL Profiling".

    By 2017 I found yet another type of tools of enormous value for my daily work in Support - profilers (not only pt-pmp, but also real ones), specifically perf in case of any modern Linux. I had found out (hard way) that proper instrumentation of source code, while useful as MySQL Performance Schema (and Oracle's wait events long before it) shown, is never 100% complete and ready to use in practice, unlike proper OS level profiling tools that Linux got since kernel 2.6.x. Profilers helped me to solve real life performance problems and my findings were later summarized in this FOSDEM 2017 talk, the best one I ever made IMHO. It was a fruitful year for blogging and it was hard to pick up one post out of 32. I continued my gdb studies of various MySQL features and ended up with a long workshop on this topic presented in Sofia to few attendees.

  • 2018. "On Some Problematic Oracle MySQL Server Features".

    In 2018 I had crazy plans to go to Oracle Open World and speak there about problems with MySQL and Oracle's way of developing it. So, many of my numerous (37 in total) blog posts that year were explaining my views on these in details. I picked up the one above as it summarized the most problematic features of MySQL server itself (namely implementations of InnoDB data compression, FULLTEXT indexes, so called "online" DDL, automatic recalculation of InnoDB "persistent" statistics, and partitioning, as well as some details on how all these (does not) work well together). This was surely NOT a topic to be accepted for any MySQL conference! But posts from this series are still useful to review even today, as some of these features got zero improvements even in MySQL 8.0 (and just few are planned or done by other vendors, with MyRocks providing better data compression and with some ongoing changes to InnoDB that are implemented by MariaDB Corporation). I had to cancel my FOSDEM talk that year due to family issues, and had not made any public presentations.

  • 2019. "Dynamic Tracing of MySQL Server With perf probe - Basic Example".

    The year of 2019 was really successful for me. In May I've got "MySQL Community Contributor of The Year" award "for bug identification and submission", that is, for activity that is the main topic of this blog. Obviously, many (of 26 in total) blog posts were about bugs, bugs reporting and bugs processing, same as my FOSDEM talk in 2019. But later I stopped exploiting "MySQL bugs" topic and switched to my current interests (dynamic tracing and profiling with perf, BPF tools and bpftrace). So, I picked up one of the key blog posts that provided additional details for my Percona Live Europe 2019 talk on the topic.

    I had a lot of plans for related studies and talks for 2020, but with COVID-19 breaking everything and conferences cancelled it may happen so that this blog would be the only public platform for all my MySQL-related activities this year.
I also checked many old photos made with Nokia dumb phone while working on this post. Who known when (and if) I ever see the West Pier in Brighton again and swim nearby...
I do not plan to stop blogging any time soon. So, stay tuned for the next 200 post :)

Sunday, March 15, 2020

Fun with Bugs #95 - On MySQL Bug Reports I am Subscribed to, Part XXIX

With conferences cancelled or postponed and people forced to stay at home due to COVID-19 wide spreading, what can be better than to give my readers a new list of MySQL bugs to check? Useful reading should help! So today I continue my previous post with a quick review of bugs I've subscribed to in February, 2020, while things were still going as usual for most of us...

Here is the list of InnoDB, replication, optimizer and some other bugs to note while working with recent MySQL 5.7.x and 8.0.19 releases:
  • Bug #98473 - "group replication will be block after lock table". This problem report by phoenix Zhang was declared not a bug recently. Looks like for group_replication_consistency= BEFORE_AND_AFTER it is expected to get nodes blocked if one of them executed LOCK TABLE ... WRITE and another tried to insert some rows into that table. Check last comment by Nuno Carvalho for more details. Having multiple nodes that change or block data in clusters is always fun. See also Bug #98643 - "group replication will be block primary node shutdown" from the same bug reporter. Analysis is still in progress for it
  • Bug #98498 - "P_S view status_by_thread does not reflect correct results". This bug was reported by Oli Sennhauser (my former colleague in MySQL, founder of FromDual). Really weird results in the output.
  • Bug #98501 - "Exchanging partition with tables does not update SDI correctly". Having a data dictionary is cool and useful, but the information there should be consistent in all places/forms where it is stored. Fungo Wang found and reported cases when it is wrong in the individual .ibd files for partitions or imported tablespaces. After some arguing about backups tools supported, the bug was verified.
  • Bug #98511 - "OPTIMIZE TABLE on myisam can increase table size (~2x) and reduce performance".  This funny bug looks like a regression in MySQL 8.0.x comparing to 5.7. I doubt Oracle is going to fix anything for MyISAM, but maybe the regression still matters for them. As it often happens, this bug reported by Pete Dishman was verified without adding a regression tag.
  • Bug #98520 - "Suboptimal implementations for some load/store functions for little-endian arch". Alexey Kopytov identified some remaining performance problems in low level functions like int6store() and uint6korr() in MySQL 8.0 for platforms like x86_64 or ARM64. He request to optimize those functions for little-endian architectures by providing specialized implementations, as it was done in MySQL 8.0.x for many other similar and more widely used functions.
  • Bug #98530 - "crash when inplace encryption resumes makes tablespace unusable". This bug (with nice MTR test case) was reported by Satya Bodapati from Percona. See also his another bug report, Bug #98537 - "inplace encryption resume thread doesn't update DD properly".
  • Bug #98546 - "Transient indexes statistics are updated in foreground causing performance issue". As noted by Charly Batista, if persistent statistics is not enabled InnoDB checks if 1/16 rows of the table have been changed and if this was the case, it calls the dict_stats_update function in the foreground. Moreover, it does not only degrade performance while recalculating stats in the foreground thread, but also sets RW_X_LATCH for the table in the process to serialize all access. So, use persistent statistics, but do not let it be automatically recalculated (and here I explained why).
  • Bug #98616 - "XA PREPARE/XA COMMIT/XA ROLLBACK lost if mysql crash just after binlog flush". Yet another problem with XA transactions support in MySQL. Dennis Gao provided a patch and kindly explained that currently:
    "When mysql start-up recover, the innodb engine will only recover the transaction in prepared state, which means the undo->state of the recovered transaction must be TRX_UNDO_PREPARED (check trx_rollback_resurrected in So if a "xa prepare" transaction only flush binlog, it will just be rollback during start-up recover and lost."
  • Bug #98624 - "Can't connect to MySQL after establishing around 65536 connections". Yet another great finding by Fungo Wang. Basically, this is a bug in the MDL subsystem, the scalability of MDL is limited to 65536 users, due to the pins number limitation (LF_PINBOX_MAX_PINS = 65536) of the LF_HASH it employs.
  • Bug #98639 - "Redundant row-level locking for secondary index". I think this simple case pointed out by Sergei Muraviev is a yet another case of excessive locking where improvement is possible. I do not agree with simple "Not a bug" status.
  • Bug #98642 - "CONSISTENT SNAPSHOT CAN BE CORRUPTED BY OTHER TRANSACTIONS". This is a really serious bug found by Erwan MAS. It took me some efforts to make sure this bug report was properly processed and verified (my comment in the bug report is hidden, but that does not matter as long as it was really treated seriously). Looks like MySQL 8.0.x may be not affected, but recent 5.7.x and 5.6.x versions are affected for sure. Take care!
  • Bug #98651 - "Inserting into temporary table from function breaks replication in MIXED mode". This regression bug in MySQL 8 (vs 5.7.29) was reported by Alexey Gavrilov, who had create a separate GitHub repository for the test case.
  • Bug #98665 - "replication broken on blackhole node if binlog_rows_query_log_events on master". This bug was reported by Zhenghu Wen. Take care if you use BLACKHOLE tables on slaves. Both MySQL 8.0.19 and 5.7.29 are affected.
  • Bug #98673 - "Allow hints to reference query block by system name". I like optimizer hints implementation in MySQL. This feature request to make them even more useful and less confusing in case of complex queries. As Kaiwang CHen put it:
    "Note that the query blocks are internally identified with a number
    (SELECT_LEX::select_number), with which system names are defined. That
    system name could be explored to refer to any query block in the query."
    I do not see a patch in the bug report, but there was a plan to provide it.
I have few more bugs in my list for February, but let's continue with them next time. We have many boring weeks ahead it seems...

Frida is always searching for something... I do the same with MySQL bugs.
To summarize:
  1. Group replication still has a lot of problems to resolve before becoming a really mature solution. Ask Galera developers :) 
  2. There are regressions in MySQL 8.0.19.
  3. Percona and Alibaba engineers still help to make MySQL better.
  4. I still do not see a consistent use of "regression" tag for verified regression bugs. This is unfortunate.
  5. InnoDB locking still needs more attention.
  6. There is too much Docker usage in the industry for my linking...

Saturday, February 22, 2020

Fun with Bugs #94 - On MySQL Bug Reports I am Subscribed to, Part XXVIII

I may get a chance to speak about proper bugs processing for open source projects later this year, so I have to keep reviewing recent MySQL bugs to be ready for that. In my previous post in this series I listed some interesting MySQL bug reports created in December, 2019. Time to move on to January, 2020! Belated Happy New Year of cool MySQL Bugs!

As usual I mostly care about InnoDB, replication and optimizer bugs and explicitly mention bug reporter by name and give link to his other active reports (if any). I also pick up examples of proper (or improper) reporter and Oracle engineers attitudes. Here is the list:
  • Bug #98103 - "unexpected behavior while logging an aborted query in the slow query log".  Query that was killed while waiting for the table metadata lock is not only get logged, but also lock wait time is saved as query execution time. I'd like to highlight how bug reporter, Pranay Motupalli, used gdb to study what really happens in the code in this case. Perfect bug report!
  • Bug #98113 - "Crash possible when load & unload a connection handler". The (quite obvious) bug was verified based on code review, but only after some efforts were spent by Oracle engineer on denial to accept the problem and its importance. This bug was reported by Fangxin Flou.
  • Bug #98132 - "Analyze table leads to empty statistics during online rebuild DDL ". Nice addition to my collections! This bug with a nice and clear test case was reported by Albert Hu, who also suggested a fix.
  • Bug #98139 - "Committing a XA transaction causes a wrong sequence of events in binlog". This bug reported by Dehao Wang was verified as a "documentation" one, but I doubt documenting current behavior properly is an acceptable fix. Bug reporter suggested to commit in the binary log first, for example. Current implementation that allows users to commit/rollback a XA transaction by using another connection if the former connection is closed or killed, is risky. A lot of arguing happened in comments in the process, and my comment asking for a clear quote from the manual:
    Would you be so kind to share some text from this page you mentioned:

    or any other fine MySQL 8 manual page stating that XA COMMIT is NOT supported when executed from session/connection/thread other than those prepared the XA transaction? I am doing something wrong probably, but I can not find such text anywhere.
    was hidden. Let's see what happens to this bug report next.
  • Bug #98211 - "Auto increment value didn't reset correctly.". Not sure what this bug reported by Zhao Jianwei has to do with "Data Types", IMHO it's more about DDL or data dictionary. Again, some sarcastic comments from Community users were needed to put work on this bug back on track...
  • Bug #98220 - "with log_slow_extra=on Errno: info not getting updated correctly for error". This bug was reported by lalit Choudhary from Percona.
  • Bug #98227 - "innodb_stats_method='nulls_ignored' and persistent stats get wrong cardinalities". I think category is wrong for this bug. It's a but in InnoDB's persistent statistics implementation, one of many. The bug was reported by Agustín G from Percona.
  • Bug #98231 - "show index from a partition table gets a wrong cardinality value". Yet another by report by Albert Hu. that ended up as a "documentation" bug for now, even though older MySQL versions provided better cardinality estimations than MySQL 8.0 in this case (so this is a regression of a kind). I hope the bug will be re-classified and properly processed later.
  • Bug #98238 - "I_S.KEY_COLUMN_USAGE is very slow". I am surprised to see such a bug in MySQL 8. According to the bug reporter, Manuel Mausz, this is also a kind of regression comparing to older MySQL version, where these queries used to run faster. Surely, no "regression" tag in this case was added.
  • Bug #98284 - "Low sysbench score in the case of a large number of connections". This notable performance regression of MySQL 8 vs 5.7 was reported by zanye zjy. perf profiling pointed out towards ppoll() where a lot of time is spent. There is a fix suggested by Fangxin Flou (to use poll() instead), but the bug is still "Open".
  • Bug #98287 - "Explanation of hash joins is inconsistent across EXPLAIN formats". This bug was reported by Saverio M and ended up marked as a duplicate of Bug #97299 fixed in upcoming 8.0.20. Use EXPLAIN FORMAT=TREE in the meantime to see proper information about hash joins usage in the plan.
  • Bug #98288 - "xa commit crash lead mysql replication error". This bug report from Phoenix Zhang (who also suggested a patch) was declared a duplicate of Bug #76233 - "XA prepare is logged ahead of engine prepare" (that I've already discussed among other XA transactions bugs here).
  • Bug #98324 - "Deadlocks more frequent since version 5.7.26". Nice regression bug report by Przemyslaw Malkowski from Percona, with additional test provided later by Stephen Wei . Interestingly enough, test results shared by Umesh Shastry show that MySQL 8.0.19 is affected in the same way as 5.7.26+, but 8.0.19 is NOT listed as one of versions affected. This is a mistake to fix, along with missing regression tag.
  • Bug #98427 - "InnoDB FullText AUX Tables are broken in 8.0". Yet another regression in MySQL 8 was found by Satya Bodapati. Change in default collation for utf8mb4 character set caused this it seems. InnoDB FULLTEXT search was far from perfect anyway...
The are clouds in the sky of MySQL bugs processing.
To summarize:
  1.  Still too much time and efforts are sometimes spent on arguing with bug reporter instead of accepting and processing bugs properly. This is unfortunate.
  2. Sometimes bugs are wrongly classified when verified (documentation vs code bug, wrong category, wrong severity, not all affected versions are listed, ignoring regression etc). This is also unfortunate.
  3. Percona engineers still help to make MySQL better.
  4. There are some fixes in upcoming MySQL 8.0.20 that I am waiting for :)
  5. XA transactions in MySQL are badly broken (they are not atomic in storage engine + binary log) and hardly safe to use in reality.

Saturday, February 8, 2020

Fun with Bugs #93 - On MySQL Bug Reports I am Subscribed to, Part XXVII

No matter what I write and present about dynamic tracing, blog posts about MySQL bugs are more popular based on statistics. So, to make more readers happy, I'd like to continue my review of interesting bugs reported in November with this post on bugs reported during December, 2019.

As usual, I'll try to concentrate on bug reports related to InnoDB, replication and optimizer, but some other categories also got my attention:
  • Bug #97911 - "[FATAL] Semaphore wait has lasted > 600 seconds. We intentionally crash the serv...". This bug got marked as a duplicate of other, older long semaphore wait bug (in "Can't repeat" status!) without much analysis. I think all Oracle engineers who added comments to that bug missed one interesting point:
    ... [ERROR] [MY-012872] [InnoDB] Semaphore wait has lasted > 39052544 seconds.
    even though bug reporter highlighted it in a comment. Reported wait time is a problem and is surely a bug, no matter how to reproduce the long wait itself and what is its root cause!
  • Bug #97913 - "Undo logs growing during partitioning ALTER queries". This bug (affecting only MySQL 5.7.x) was reported by Przemyslaw Malkowski from Percona, who also presented useful examples of monitoring queries to the information_schema.innodb_metrics and performance_schema. Check also comments that may explain why 8.0 is not affected in a similar way.
  • Bug #97935 - "Memory leak in client connection using information_schema". It took some efforts (starting from but not limited to Valgrind Massif profiling of heap memory usage) and time for Daniel Nichter to prove the point and get this bug "Verified". It is also not clear if MySQL 8 is also affected.
  • Bug #97950 - "buf_read_page_handle_error can trigger assert failure". Bug reporter, Shu Lin, tried his best to make the point. It's clear enough how to repeat this, and one could use one of documented test synchonisation methods if gdb is too much for bug verification. I do not think this bug was handled properly or got the level of attention it truly deserved.
  • Bug #97966 - "XA COMMIT in another session will not write binlog event". This bug was reported by Lujun Wang and immediately verified, but again with no documented check if MySQL 8 is affected. This happens too often, unfortunately.
  • Bug #97971 - "Roles not handling column level privileges correctly; Can SELECT, but not UPDATE". Clear and simple bug report with a workaround from Travis Bement. It was immediately verified.
  • Bug #98014 - "Lossy implicit conversion in conditional breaks ONLY_FULL_GROUP_BY". Yet another case of (IMHO) improper bug processing. The argument presented (from the manual):
    "MySQL 5.7.5 and later also permits a nonaggregate column not named in a GROUP BY clause when ONLY_FULL_GROUP_BY SQL mode is enabled, provided that this column is limited to a single value"
    does not apply, as "single value" for = 0 is NOT selected, we have multiple Host values matching it due to conversion. This is how proper version (guess what it is) works:
    mysql> SELECT User, Host, COUNT(*) FROM mysql.user WHERE Host = 0 GROUP BY 1;
    ERROR 1055 (42000): 'mysql.user.Host' isn't in GROUP BY
    mysql> select @@sql_mode;
    | @@sql_mode         |
    1 row in set (0.001 sec)

    mysql> set session sql_mode='';
    Query OK, 0 rows affected (0.029 sec)

    mysql> SELECT User, Host, COUNT(*) FROM mysql.user WHERE Host = 0 GROUP BY 1;+---------------+-----------+----------+
    | User          | Host      | COUNT(*) |
    | data_engineer |           |        1 |
    | en            | localhost |        1 |
    | ro1           |           |        1 |
    | ro2           |           |        1 |
    | role-1        |           |        1 |
    | root          | ::1       |        3 |
    | user1         | %         |        1 |
    13 rows in set, 17 warnings (0.003 sec)
    I think this bug reported by Joshua Varner must be verified.
  • Bug #98046 - "Inconsistent behavior while logging a killed query in the slow query log". Bug reporter, Pranay Motupalli, provided a clear test case and a detailed analysis, including the gdb debugging session that proves the point. Nice bug report.
  • Bug #98055 - "MySQL Optimizer Bug not picking right index". Both the bug reporter (Sudheer Gadipathi) and engineer who verified the bug stated that MySQL 8.0.x is similarly affected (UNIQUE key is preferred for the partitioned table, even though there is a better non-unique index). But 8.0.x is NOT listed in the "Version:" filed. Weird.
  • Bug #98068 - "SELECT FOR UPDATE not-exist table in PERFORMANCE SCHEMA reports confusing error". This is a funny (but still a regression) bug report by William ZHANG. Proper versions work like this:
    mysql> select database();+--------------------+
    | database()         |
    | performance_schema |
    1 row in set (0.001 sec)

    mysql> select * from not_exist_table;
    ERROR 1146 (42S02): Table 'performance_schema.not_exist_table' doesn't exist
    mysql> select * from not_exist_table for update;
    ERROR 1146 (42S02): Table 'performance_schema.not_exist_table' doesn't exist
  • Bug #98072 - "innochecksum summary shows blob pages as other type of page for 8.0 tables". The bug was reported by SERGEY KUZMICHEV. This time "regression" tag is missing, even though it's clearly stated that MySQL 5.7 worked differently. This is from the proper version:
    ================PAGE TYPE SUMMARY==============
           1        Index page
           0        Undo log page
           1        Inode page
           0        Insert buffer free list page
         508        Freshly allocated page
           1        Insert buffer bitmap
           0        System page
           0        Transaction system page
           1        File Space Header
           0        Extent descriptor page
          64        BLOB page
           0        Compressed BLOB page
           0        Page compressed page
           0        Page compressed encrypted page
           0        Other type of page

  • Bug #98083 - "Restarting the computer when deleting the database will cause directory residues". One would expect that MySQL 8 with a data dictionary should have some means to figure out the remaining database directory for a dropped database upon startup (as it stores information about databases elsewhere) and do proper cleanup. I think this bug reported by Jinming Liao must be verified and fixed. There is no "... manual creation or deletion of tables or databases..." involved in this case.
  • Bug #98091 - "InnoDB does not initialize raw disk partitions". As simple as that and both 5.7.29 and 8.0.219 are surely affected. It was not always the case, I've used raw devices myself with older MySQL versions, so this bug reported by Saverio M is a regression. Still, "regression" tag is missing.
That's all bugs reported in December, 2019 that I cared to subscribe to  and mention here. Next time I'll check bugs reported in January, 2020. There are at least 16 in my list already, so stay tuned.

Follow the links in this post to get more details about profiling and creating off-CPU FlameGraphs for MySQL. This post is devoted to bugs, though :)

To summarize:
  1. I am happy to see bug reports from people whom I never noticed before. MySQL Community is alive.
  2. Some flexibility in following common sense based bugs verification procedures is still visible. Bugs reported for 5.7 are not checked on 8.0 (or the results of this check are not documented in public), nobody cares to read what bug reporter says carefully or go extra mile, "regression" tag not added, and so on. 
  3. Probably at this stage my writings are mostly ignored by Oracle's decision makers. But I keep watching them all anyway.

Sunday, January 26, 2020

Using bpftrace on Fedora 29 - More Advanced Examples

It happened so that I did not have much time to devote to further eBPF and bpftrace-based tracing and profiling since October 2019, when I posted a simple example of how it can be used on Fedora 29 to "emulate" slow query log (to some extent, by recording queries and time it took to execute them) with dynamic probes. Recently I started preparing for my talk about ftrace, eBPF and bpftrace planned for FOSDEM MySQL, MariaDB and Friends Devroom, but rejected there and later accepted for "MariaDB Day Brussels 0202 2020" (that I suggest everyone to attend, at least if you do not have better plans for the second day of FOSDEM 2020).

So, I decided to check for any ways to use bpftrace as a quick profiler, maybe "embed" it into pt-pmp one day and, surely, get what MariaDB developers ask me most often while working on performance problems or hangs - stack traces. This blog post describes my "achievements" so far on this way.

I had the same version of bpftrace coming from Fedora RPM (that had not updated in the meantime):
[openxs@fc29 server]$ rpm -q -a | grep bpftrace
So, after checking this fine reference guide quickly, I came up with a simple one-liner:
[openxs@fc29 ~]$ sudo bpftrace -e 'profile:hz:99 { @[ustack] = count(); }'
Error creating map: '@'
Error creating stack id map
This might have happened because kernel.perf_event_max_stackis smaller than 127. Try to tweak this value with sysctl kernel.perf_event_max_stack=<new value>
Error creating perf event map (-1)
Attaching 1 probe...
Segmentation fault
Ups! I tried to do what error message suggested:
[openxs@fc29 ~]$ sudo sysctl -w kernel.perf_event_max_stack=1000
kernel.perf_event_max_stack = 1000
But it had not helped. This is how I hit my first problem with bpftrace. Fedora had not cared to update it when it stopped matching "something" in a new kernel, for whatever reason. This is actually a real problem for eBPF programs - what they assume may not be "there" in a kernel, so they have to be checked carefully. There is a separate "BPF Type Format (BTF)" for encoding the debug info related to BPF program (or map) that may help to find out if the program is "compatible" without trying it. In my case incompatibility was obvious and I needed some fast way to proceed with bpftrace, in a hope to come up with some useful examples for my talk.

So, i decided to build new version of bpftrace from recent GitHub source (against bcc tools I had installed and supposedly up to date). This ended up with errors no matter what I tried:
[openxs@fc29 build]$ make
[  3%] Built target parser
[  4%] Built target arch
[  5%] Building CXX object src/ast/CMakeFiles/ast.dir/irbuilderbpf.cpp.o
/mnt/home/openxs/git/bpftrace/src/ast/irbuilderbpf.cpp: In member function 'void bpftrace::ast::IRBuilderBPF::CreateSignal(llvm::Value*)':
/mnt/home/openxs/git/bpftrace/src/ast/irbuilderbpf.cpp:669:16: error: 'BPF_FUNC_send_signal' was not declared in this scope
Quick search led me to the idea that it's actually a know and recently reported problem, see Issue #1014. As a solution I decided to rebuild bcc tools from GitHub source as well (the alternative to to change bpftrace source) and removed Fedora packages. To make a long story short, this is what I did eventually (from fc -l output):
1051     rm -rf bcc/
1052     rm -rf bpftrace/
1053     sudo dnf remove bcc
1054     git clone
1055     cd bcc/
1056     git submodule update --init --recursive
1057     sudo dnf install -y bison cmake ethtool flex git iperf libstdc++-static   python-netaddr python-pip gcc gcc-c++ make zlib-devel   elfutils-libelf-devel
1058     sudo dnf install -y luajit luajit-devel
1059     sudo dnf install -y
1060     sudo pip install pyroute2
1061     sudo dnf install -y clang clang-devel llvm llvm-devel llvm-static ncurses-devel
1062     mkdir build
1063     cd build/
1064     cmake .. -DCMAKE_INSTALL_PREFIX=/usr
1065     make
1066     make test
1067     sudo make install
1068     cd ../..
1069     git clone
1070     cd bpftrace/
1071     git submodule update --init --recursive
1072     mkdir build
1073     cd build/
1074     cmake -DCMAKE_BUILD_TYPE=Release ../
1075     make
1076     ./tests/bpftrace_test
1077     fc -l
Basically I did it all as documented in related projects for my version of Fedora. Step highlighted (1056 above) is essential though, as it was missed here in any obvious way as a mandatory (while it is in recent versions). Step 1076 was successful:
[----------] Global test environment tear-down
[==========] 350 tests from 9 test cases ran. (2590 ms total)
[  PASSED  ] 350 tests.

[openxs@fc29 build]$ make testRunning tests...
Test project /mnt/home/openxs/git/bpftrace/build
    Start 1: bpftrace_test
1/3 Test #1: bpftrace_test ....................   Passed    2.42 sec
    Start 2: runtime_test
2/3 Test #2: runtime_test .....................***Failed   21.10 sec
    Start 3: tools-parsing-test
3/3 Test #3: tools-parsing-test ...............***Failed    5.88 sec

33% tests passed, 2 tests failed out of 3

Total Test time (real) =  29.41 sec

The following tests FAILED:
          2 - runtime_test (Failed)
          3 - tools-parsing-test (Failed)
Errors while running CTest
make: *** [Makefile:128: test] Error 8
while make test shown some failures (same as make test for bcc tools), but form previous experience this had not looked like a big deal. I continued basic testing that the resulting bpftrace now works (I tried to add kernel probe, guess what software runs on this workstation):
[openxs@fc29 build]$ sudo ./src/bpftrace -e 'kprobe:do_nanosleep { printf("sleep by %s\n", comm); }'
Attaching 1 probe...
sleep by percona-qan-age
sleep by percona-qan-age
sleep by skypeforlinux
sleep by mysqld
So, with some confidence built up, I've installed my new and very recent bpftrace tool I built myself:
[openxs@fc29 build]$ sudo make install
[  3%] Built target parser
-- Installing: /usr/local/bin/bpftrace
-- Installing: /usr/local/man/man8/xfsdist.8.gz

[openxs@fc29 build]$ which bpftrace

[openxs@fc29 build]$ cd ..
[openxs@fc29 bpftrace]$ git log -1
commit afafbf561522dd33fa316be3e33375bc662399ac (HEAD -> master, origin/master, origin/HEAD)
Author: bas smit <>
Date:   Fri Jan 17 12:33:43 2020 +0100

    Add `--info` flag

    This prints the supported kernel features
Quick test involved starting MariaDB 10.3 with thread pool enabled (more on "why thread pool?" in some next blog post):
[openxs@fc29 maria10.3]$ ./bin/mysqld_safe --no-defaults --thread_handling=pool-of-threads --thread_pool_idle_timeout=10 &
[1] 32643
and running sysbench:
[openxs@fc29 maria10.3]$ sysbench /usr/local/share/sysbench/oltp_update_index.lua --mysql-host= --mysql-user=root --mysql-port=3306 --threads=10 --tables=4 --table-size=1000000 --time=600 --report-interval=5 run
sysbench 1.1.0-174f3aa (using bundled LuaJIT 2.1.0-beta2)

Running the test with following options:
Number of threads: 10
Report intermediate results every 5 second(s)
Initializing random number generator from current time

Initializing worker threads...

Threads started!

[ 5s ] thds: 10 tps: 54.37 qps: 54.37 (r/w/o: 0.00/54.37/0.00) lat (ms,95%): 467.30 err/s: 0.00 reconn/s: 0.00
[ 10s ] thds: 10 tps: 40.60 qps: 40.60 (r/w/o: 0.00/40.60/0.00) lat (ms,95%): 530.08 err/s: 0.00 reconn/s: 0.00
[ 15s ] thds: 10 tps: 40.00 qps: 40.00 (r/w/o: 0.00/40.00/0.00) lat (ms,95%): 520.62 err/s: 0.00 reconn/s: 0.00
[ 20s ] thds: 10 tps: 38.00 qps: 38.00 (r/w/o: 0.00/38.00/0.00) lat (ms,95%): 623.33 err/s: 0.00 reconn/s: 0.00
[ 25s ] thds: 10 tps: 34.00 qps: 34.00 (r/w/o: 0.00/34.00/0.00) lat (ms,95%): 746.32 err/s: 0.00 reconn/s: 0.00
[ 30s ] thds: 10 tps: 36.40 qps: 36.40 (r/w/o: 0.00/36.40/0.00) lat (ms,95%): 549.52 err/s: 0.00 reconn/s: 0.00
[ 35s ] thds: 10 tps: 37.80 qps: 37.80 (r/w/o: 0.00/37.80/0.00) lat (ms,95%): 601.29 err/s: 0.00 reconn/s: 0.00
and adding the probe like this in another shell:
[openxs@fc29 ~]$ sudo bpftrace -e 'profile:hz:99 { @[ustack] = count(); }' > /tmp/bpf.stack
When I stopped collecting with Ctrl-C, bpftrace printed the content of associative arrays collected (with unique user space stack 9for all processes!) as index and count of time this stack was as a value). No segmentation fault, unlike with the version from package! In the resulting file we can see (among other things):
[openxs@fc29 bpftrace]$ cat /tmp/bpf.stack | more
Attaching 1 probe...

    rec_get_offsets_func(unsigned char const*, dict_index_t const*, unsigned short*, bool, unsigned long, mem_block_info_t**)+31
    page_cur_search_with_match_bytes(buf_block_t const*, dict_index_t const*, dtuple_t const*, page_cur_mode_t, unsigned long*, unsigned long*, unsigned long*, unsigned long*, page_cur_t*)+1773
    btr_cur_search_to_nth_level_func(dict_index_t*, unsigned long, dtuple_t const*, page_cur_mode_t, unsigned long, btr_cur_t*, rw_lock_t*, char const*, unsigned int, mtr_t*, unsigned long)+8316
    btr_pcur_open_low(dict_index_t*, unsigned long, dtuple_t const*, page_cur_mode_t, unsigned long, btr_pcur_t*, char const*, unsigned int, unsigned long, mtr_t*) [clone .constprop.33]+146
    row_search_on_row_ref(btr_pcur_t*, unsigned long, dict_table_t const*, dtuple_t const*, mtr_t*)+85
    row_purge_poss_sec(purge_node_t*, dict_index_t*, dtuple_t const*, btr_pcur_t*, mtr_t*, bool)+503
    row_purge_remove_sec_if_poss_leaf(purge_node_t*, dict_index_t*, dtuple_t const*)+1293
    row_purge_record_func(purge_node_t*, unsigned char*, que_thr_t const*, bool)+2271
]: 1


    rec_get_offsets_func(unsigned char const*, dict_index_t const*, unsigned short*, bool, unsigned long, mem_block_info_t**)+31
    btr_search_build_page_hash_index(dict_index_t*, buf_block_t*, rw_lock_t*, unsigned long, unsigned long, unsigned long)+2781
    btr_search_info_update_slow(btr_search_t*, btr_cur_t*)+622
    btr_cur_search_to_nth_level_func(dict_index_t*, unsigned long, dtuple_t const*, page_cur_mode_t, unsigned long, btr_cur_t*, rw_lock_t*, char const*, unsigned int, mtr_t*, unsigned long)+10130
    row_search_mvcc(unsigned char*, page_cur_mode_t, row_prebuilt_t*, unsigned long, unsigned long)+4823
    ha_innobase::index_read(unsigned char*, unsigned char const*, unsigned int, ha_rkey_function)+338
    handler::ha_index_read_map(unsigned char*, unsigned char const*, unsigned long, ha_rkey_function)+280
    handler::read_range_first(st_key_range const*, st_key_range const*, bool, bool)+98
    mysql_update(THD*, TABLE_LIST*, List<Item>&, List<Item>&, Item*, unsigned int, st_order*, unsigned long long, bool, unsigned long long*, unsigned long long*)+3560
    Prepared_statement::execute(String*, bool)+979
    Prepared_statement::execute_loop(String*, bool, unsigned char*, unsigned char*)+154
    mysql_stmt_execute_common(THD*, unsigned long, unsigned char*, unsigned char*, unsigned long, bool, bool)+349
    mysqld_stmt_execute(THD*, char*, unsigned int)+37
    dispatch_command(enum_server_command, THD*, char*, unsigned int, bool, bool)+5123
]: 1


    trx_purge(unsigned long, bool)+1054
]: 2
    ut_crc32_sw(unsigned char const*, unsigned long)+1611
    buf_calc_page_crc32(unsigned char const*)+61
    buf_flush_init_for_writing(buf_block_t const*, unsigned char*, void*, unsigned long)+872
    buf_flush_write_block_low(buf_page_t*, buf_flush_t, bool)+149
    buf_flush_page(buf_pool_t*, buf_page_t*, buf_flush_t, bool)+607
    buf_flush_page_and_try_neighbors(buf_page_t*, buf_flush_t, unsigned long, unsigned long*)+1627
    buf_do_flush_list_batch(buf_pool_t*, unsigned long, unsigned long)+392
]: 2

@[]: 6474
So, this is the default format to output stacks (one line per call, no address, frame number or anything, symbols properly resolved for my MariaDB built from GitHub source etc). But I had something like this mind eventually (from pt-pmp):
[openxs@fc29 bpftrace]$ pt-pmp
Tue Jan 21 13:52:12 EET 2020

     10 syscall(,__io_getevents_0_4(,LinuxAIOHandler::collect(,LinuxAIOHandler::poll(,os_aio_linux_handler(stl_vector.h:805),os_aio_handler(stl_vector.h:805),fil_aio_wait(,io_handler_thread(,start_thread(,clone(
      6 pthread_cond_wait,os_event::wait(sync0types.h:476),os_event::wait_low(sync0types.h:476),log_write_up_to(,trx_flush_log_if_needed_low(,trx_flush_log_if_needed(,trx_commit_complete_for_mysql(,innobase_commit(,commit_one_phase_2(,ha_commit_one_phase(,ha_commit_trans(,trans_commit_stmt(,mysql_execute_command(,Prepared_statement::execute(,Prepared_statement::execute_loop(,Prepared_statement::execute_loop(,mysql_stmt_execute_common(,mysqld_stmt_execute(,dispatch_command(,do_command(,threadpool_process_request(,tp_callback(,worker_main(,start_thread(,clone(
      3 pthread_cond_wait,os_event::wait(sync0types.h:476),os_event::wait_low(sync0types.h:476),srv_resume_thread(,srv_worker_thread(,start_thread(,clone(
      3 pthread_cond_timedwait,inline_mysql_cond_timedwait(mysql_thread.h:1215),get_event(mysql_thread.h:1215),worker_main(,start_thread(,clone(
That is, number of times collapsed stack was noted and then the stack itself, without function arguments if possible. This kind of stack is generated from gdb outputs that looks as follows:
Thread 1 (Thread 0x7f29eee66900 (LWP 32728)):
#0  0x00007f29ef25f431 in poll () from /lib64/
#1  0x000055cc4ece656f in poll (__timeout=-1, __nfds=2, __fds=0x7ffe174108e0) at /usr/include/bits/poll2.h:41
#2  handle_connections_sockets () at /mnt/home/openxs/git/server/sql/
#3  0x000055cc4ece7a7c in mysqld_main (argc=<optimized out>, argv=<optimized out>) at /mnt/home/openxs/git/server/sql/
#4  0x00007f29ef190413 in __libc_start_main () from /lib64/
#5  0x000055cc4ecd9ece in _start () at /mnt/home/openxs/git/server/sql/sql_list.h:158
So, each tread stack starts from 'Thread..." line, each frame is numbered and function call is actually the fourth word in the output (in awk terms).
Based on that I tried add probe to produce stack in perf format for mysqld process, with some decoration to have a clear marker of when new capture starts:
[openxs@fc29 bpftrace]$ sudo bpftrace -e 'profile:hz:99 /comm == "mysqld"/ {printf("# %s\n", ustack(perf));}' > /tmp/ustack.txt
If you wonder about the details of command format, functions used etc, check the Reference Guide. The result looks as follows in hexdump -C output:
00000000  41 74 74 61 63 68 69 6e  67 20 31 20 70 72 6f 62  |Attaching 1 prob|
00000010  65 2e 2e 2e 0a 23 20 0a  09 37 66 32 39 65 66 62  |e....# ..7f29efb|
00000020  38 34 33 30 30 20 6e 61  6e 6f 73 6c 65 65 70 2b  |84300 nanosleep+|
00000030  36 34 20 28 2f 75 73 72  2f 6c 69 62 36 34 2f 6c  |64 (/usr/lib64/l|
00000040  69 62 70 74 68 72 65 61  64 2d 32 2e 32 38 2e 73  |ibpthread-2.28.s|
00000050  6f 29 0a 0a 23 20 0a 09  35 35 63 63 34 66 32 30  |o)..# ..55cc4f20|
This is how you can see where are the new lines, how many etc, for proper parsing. Next step was to check pt-pmp source code and, review and adapt some awk magic used there to collapse stacks, get rid of function arguments etc, and I've ended up with this draft version:
[openxs@fc29 maria10.3]$ cat /tmp/ustack.txt | awk '
BEGIN { s = ""; }
/^\#/ { print s; s = ""; }
/^\t/ { if (index($2, "(") > 0) {targ = substr($2, 1, index($2, "(") - 1)} else {targ = substr($2, 1, index($2, "+") - 1)} ; if (s != "") { s = s "," targ } else { s = targ } }
END { print s }' | sort | uniq -c | sort -r -n -k 1,1 | more

    199 __sched_yield,srv_purge_coordinator_thread,start_thread
     16 srv_get_task_queue_length,trx_purge,srv_purge_coordinator_thread,start_thread
     13 trx_purge,srv_purge_coordinator_thread,start_thread
      4 srv_purge_coordinator_thread,start_thread
      4 srv_get_task_queue_length,srv_purge_coordinator_thread,start_thread
      2 ut_crc32_sw,buf_calc_page_crc32,buf_page_is_corrupted,buf_page_io_complete,buf_read_page,buf_page_get_gen,btr_cur_search_to_nth_level_func,row_search_mvcc,ha_innobase::index_read,handler::ha_index_read_map,handler::read_range_first,handler::multi_range_read_next,Mrr_simple_index_reader::get_next,DsMrr_impl::dsmrr_next,QUICK_RANGE_SELECT::get_next,rr_quick,mysql_update,mysql_execute_command,Prepared_statement::execute,Prepared_statement::execute_loop,mysql_stmt_execute_common,mysqld_stmt_execute,dispatch_command,do_command,tp_callback,worker_main,start_thread
      2 syscall,
      2 __pwrite,
      2 mtr_t::commit,row_ins_sec_index_entry_low,row_ins_sec_index_entry,row_upd_sec_index_entry,row_upd_step,row_update_for_mysql,ha_innobase::update_row,handler::ha_update_row,mysql_update,mysql_execute_command,Prepared_statement::execute,Prepared_statement::execute_loop,mysql_stmt_execute_common,mysqld_stmt_execute,dispatch_command,do_command,tp_callback,worker_main,start_thread
The above looks reasonable (even if not yet a direct match to what pt-pmp produces, I am too lazy to do much awk these days).

What's more interesting is that with bpftrace we can also get kernel stack for thread while profiling. I took a quick look at what we may get as a result of combining both:
[openxs@fc29 bpftrace]$ sudo bpftrace -e 'profile:hz:99 /comm == "mysqld"/ {printf("# %s\n%s\n", kstack(perf), ustack(perf));}' > /tmp/kstack.txt
[sudo] password for openxs:
I just pring one after the other without any separator in perf format, so I can apply the same trick as above to the result:
openxs@fc29 maria10.3]$ cat /tmp/kstack.txt | awk '                   BEGIN { s = ""; }
/^\#/ { print s; s = ""; }
/^\t/ { if (index($2, "(") > 0) {targ = substr($2, 1, index($2, "(") - 1)} else {targ = substr($2, 1, index($2, "+") - 1)} ; if (s != "") { s = s "," targ } else { s = targ } }
END { print s }' | sort | uniq -c | sort -r -n -k 1,1 | more

     93 __sched_text_start,schedule,__x64_sys_sched_yield,do_syscall_64,entry_SYSCALL_64_after_hwframe,__sched_yield,srv_purge_coordinator_thread,start_thread
     49 __sched_yield,srv_purge_coordinator_thread,start_thread
     42 do_syscall_64,entry_SYSCALL_64_after_hwframe,__sched_yield,srv_purge_coordinator_thread,start_thread
     20 srv_get_task_queue_length,trx_purge,srv_purge_coordinator_thread,start_thread
     11 trx_purge,srv_purge_coordinator_thread,start_thread
      2 srv_get_task_queue_length,srv_purge_coordinator_thread,start_thread
      1 __x86_indirect_thunk_rax,do_syscall_64,entry_SYSCALL_64_after_hwframe,__sched_yield,srv_purge_coordinator_thread,start_thread
      1 ut_crc32_sw,log_block_calc_checksum_crc32,log_write_up_to,trx_commit_complete_for_mysql,innobase_commit,commit_one_phase_2,ha_commit_trans,trans_commit_stmt,mysql_execute_command,Prepared_statement::execute,Prepared_statement::execute_loop,mysql_stmt_execute_common,mysqld_stmt_execute,dispatch_command,do_command,tp_callback,worker_main,start_thread
      1 _raw_spin_unlock_irqrestore,ata_scsi_queuecmd,scsi_queue_rq,blk_mq_dispatch_rq_list,blk_mq_do_dispatch_sched,blk_mq_sched_dispatch_requests,__blk_mq_run_hw_queue,__blk_mq_delay_run_hw_queue,blk_mq_run_hw_queue,blk_mq_sched_insert_requests,blk_mq_flush_plug_list,blk_flush_plug_list,blk_finish_plug,ext4_writepages,do_writepages,__filemap_fdatawrite_range,file_write_and_wait_range,ext4_sync_file,do_fsync,__x64_sys_fsync,do_syscall_64,entry_SYSCALL_64_after_hwframe,fsync,fil_flush_low,fil_flush_file_spaces,buf_dblwr_update,buf_page_io_complete,fil_aio_wait,io_handler_thread,start_thread
      1 page_simple_validate_new,buf_dblwr_flush_buffered_writes,pc_flush_slot,buf_flush_page_cleaner_coordinator.cold.151,start_thread
      1 page_cur_insert_rec_write_log,page_cur_insert_rec_low,page_copy_rec_list_end_no_locks,btr_page_reorganize_low,btr_can_merge_with_page,btr_compress,btr_cur_compress_if_useful,btr_cur_pessimistic_delete,row_purge_remove_sec_if_poss_tree,row_purge_record_func,row_purge_step,que_run_threads,srv_worker_thread,start_thread
      1 os_thread_yield,srv_purge_coordinator_thread,start_thread
      1 os_thread_get_curr_id,fseg_free_page_func,btr_page_free,btr_compress,btr_cur_compress_if_useful,btr_cur_pessimistic_delete,row_purge_remove_sec_if_poss_tree,row_purge_record_func,row_purge_step,que_run_threads,srv_worker_thread,start_thread
Looks cool to me, we see where we spend time in kernel while on-CPU, not only in user space code.

It's sometimes scary and not that easy to use bpftrace, but you'll get there eventually, same as we got to North Greenwich via that Emirates Air Line back in 2019. The only limit is the sky!
* * *
To summarize:
  1. bpftrace and bcc tools you get from vendor packages may not work as expected, so be ready to get nightly builds etc or, as I prefer, build from source. If this is a problem better use ftrace :) 
  2. It seems getting stack traces with bpftrace for pt-pmp is an easy next stap to make. I may be integrated as one of tracers supported here.
  3. Now we can also add kernel stack to the picure.
  4. "JUST DO BPF"!
I have many more details to share about dynamic tracing for MySQL and MariaDB DBAs. Stay tuned!

Saturday, January 25, 2020

Dynamic Tracing of MariaDB Server With ftrace - Basic Example

Today I'd like to continue my series of blog posts about dynamic tracing of MySQL server (or Percona Server for MySQL, or MariaDB, or basically any other software, whatever you prefer) on Linux. This post ideally had to start it, as the approach discussed here is truly dynamic and suitable production servers under high load (unlike gdb tricks, most of the time), works more or less the same way starting from Linux 2.6.27, is "always there" at your fingertips (if you are root, but this is a requirement in a general case for all kinds of dynamic tracing), and does not even strictly require -debuginfo packages with symbolic information for basic usage. I mean ftrace.

To prove that this interface that appeared in 2008 is still usable, let me add the same kind of probe for tracing dispatch_command() function and printing SQL statement that somebody tries to execute (so we can avoid enabling general query log etc) with pure ftrace interface, without much explanations or background (that is presented below). In this case I use recent MariaDB Server 10.3.22 built from source on my Ubuntu 16.04 netbook (because I do not care about being shared in the feed of Planet MySQL anymore in any case).

You just have to do the following:
  1. Check that dynamic tracing with ftrace is possible:
    openxs@ao756:~$ mount | grep tracefs
    tracefs on /sys/kernel/debug/tracing type tracefs (rw,relatime)
    If you do not see tracefs mounted, mount it with the following command (as root):
    mount -t tracefs nodev /sys/kernel/tracing
  2. Find the address of (mangled) dispatch_command() function in the mysqld binary:
    openxs@ao756:~$ objdump -T /home/openxs/dbs/maria10.3/bin/mysqld | grep dispatch_command
    g    DF .text  000000000000236e  Base        _Z16dispatch_command19enum_server_commandP3THDPcjbb
    The address you need (in many cases) is the first word in the output, in hex. So, 0x0000000000587b90
  3. If you want to print values of parameters passed, you need to know how they are passed by the compiler used on the architecture used. Let me share the quote that applies to x86_64:
    "First six arguments are in rdi, rsi, rdx, rcx, r8d, r9d; remaining arguments are on the stack."
    In my case (see this post or details below, or check the source code yourself if in doubts) we need to print third argument, so it must be in the rdx (64-bit) register. For ftrace we have to use just %dx, trust me for now, I tried ;)
  4. Check probe syntax (see the picture below for quick reference) and come up with something like this to add a probe named "dc". Note that on Ubuntu 16.04 this had not worked via sudo directly, I had to become root via "sudo su -" (weird, something to check one day):
    root@ao756:~# echo 'p:dc /home/openxs/dbs/maria10.3/bin/mysqld:0x0000000000587b90 query=+0(%dx):string' > /sys/kernel/debug/tracing/uprobe_events
    All these happens while MariaDB server is running and procerssing queries, in a truly dynamic manner. There is no impact at all so far. Creators of MariaDB never added anything ot the code (or to the build process) to make this work. It's enough to have a binary that runs of Linux kernel that supports ftrace.
  5. Enable tracing for the probe added (this is when magic happens) and tracing in general:
    root@ao756:~# echo 1 > /sys/kernel/debug/tracing/events/uprobes/dc/enable
    root@ao756:~# echo 1 > /sys/kernel/debug/tracing/tracing_on
  6. Now assuming you run the following:
    root@ao756:~# mysql -uroot --socket=/tmp/mariadb.sock
    Welcome to the MySQL monitor.  Commands end with ; or \g.
    Your MySQL connection id is 92
    Server version: 5.5.5-10.3.22-MariaDB Source distribution

    Copyright (c) 2009-2019 Percona LLC and/or its affiliates
    Copyright (c) 2000, 2019, Oracle and/or its affiliates. All rights reserved.

    Oracle is a registered trademark of Oracle Corporation and/or its
    affiliates. Other names may be trademarks of their respective

    Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

    mysql> select 1+1;
    | 1+1 |
    |   2 |
    1 row in set (0.00 sec)

    mysql> select version();
    | version()       |
    | 10.3.22-MariaDB |
    1 row in set (0.00 sec)
    you may get the trace as follows, for example:
    root@ao756:~# cat /sys/kernel/debug/tracing/trace
    # tracer: nop
    # entries-in-buffer/entries-written: 9851/9851   #P:2
    #                              _-----=> irqs-off
    #                             / _----=> need-resched
    #                            | / _---=> hardirq/softirq
    #                            || / _--=> preempt-depth
    #                            ||| /     delay
    #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
    #              | |       |   ||||       |         |
              python-21369 [001] d... 209044.826580: : sys_wr: 0000000000000008
    (I had show the top of the output to give hint about format), or:
    root@ao756:~# cat /sys/kernel/debug/tracing/trace_pipe
              mysqld-1082  [000] d... 273258.971401: dc: (0x560d8a20fb90) query="select @@version_comment limit 1"
              mysqld-1082  [000] d... 273263.187839: dc: (0x560d8a20fb90) query="select 1+1"
              mysqld-1082  [001] d... 273269.128542: dc: (0x560d8a20fb90) query="select version()"
  7. If you want to stop this basic tracing, run these:
    root@ao756:~# echo 0 > /sys/kernel/debug/tracing/events/uprobes/dc/enable
    root@ao756:~# echo 0 > /sys/kernel/debug/tracing/tracing_on
So, with 7 simple steps we can enable dynamic tracing and see SQL statements executed and time when this happened.
The magic behind ftrace and steps that I really had to perform when I tried to apply this approach for the very first time will be presented in the next posts. Stay tuned!