Re: [PATCH] mm: convert mm's rss stats into percpu_counter

["Linux regression tracking #adding (Thorsten Leemhuis)" <regressions@leemhuis.info>]

[CCing the regression list, as it should be in the loop for regressions:
https://docs.kernel.org/admin-guide/reporting-regressions.html]

[TLDR: I'm adding this report to the list of tracked Linux kernel
regressions; the text you find below is based on a few templates
paragraphs you might have encountered already in similar form.
See link in footer if these mails annoy you.]

On 08.06.23 13:14, Jan Kara wrote:
> On Mon 24-10-22 05:28:41, Shakeel Butt wrote:
>> Currently mm_struct maintains rss_stats which are updated on page fault
>> and the unmapping codepaths. For page fault codepath the updates are
>> cached per thread with the batch of TASK_RSS_EVENTS_THRESH which is 64.
>> The reason for caching is performance for multithreaded applications
>> otherwise the rss_stats updates may become hotspot for such
>> applications.
>>
>> However this optimization comes with the cost of error margin in the rss
>> stats. The rss_stats for applications with large number of threads can
>> be very skewed. At worst the error margin is (nr_threads * 64) and we
>> have a lot of applications with 100s of threads, so the error margin can
>> be very high. Internally we had to reduce TASK_RSS_EVENTS_THRESH to 32.
>>
>> Recently we started seeing the unbounded errors for rss_stats for
>> specific applications which use TCP rx0cp. It seems like
>> vm_insert_pages() codepath does not sync rss_stats at all.
>>
>> This patch converts the rss_stats into percpu_counter to convert the
>> error margin from (nr_threads * 64) to approximately (nr_cpus ^ 2).
>> However this conversion enable us to get the accurate stats for
>> situations where accuracy is more important than the cpu cost. Though
>> this patch does not make such tradeoffs.
>>
>> Signed-off-by: Shakeel Butt <shakeelb@google.com>
> 
> Somewhat late to the game but our performance testing grid has noticed this
> commit causes a performance regression on shell-heavy workloads. For
> example running 'make test' in git sources on our test machine with 192
> CPUs takes about 4% longer, system time is increased by about 9%:

Thanks for the report.

I noticed this is nothing urgent. Nevertheless to be sure the issue
doesn't fall through the cracks unnoticed, I'm adding it to regzbot, the
Linux kernel regression tracking bot:

#regzbot ^introduced f1a7941243c
#regzbot title mm: performance regression on shell-heavy workloads
#regzbot backburner: not urgent according to reporter
#regzbot ignore-activity

This isn't a regression? This issue or a fix for it are already
discussed somewhere else? It was fixed already? You want to clarify when
the regression started to happen? Or point out I got the title or
something else totally wrong? Then just reply and tell me -- ideally
while also telling regzbot about it, as explained by the page listed in
the footer of this mail.

Developers: When fixing the issue, remember to add 'Link:' tags pointing
to the report (the parent of this mail). See page linked in footer for
details.

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
--
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