Re: Regression in workingset_refault latency on 5.15

* Re: Regression in workingset_refault latency on 5.15
       [not found] <CA+wXwBSyO87ZX5PVwdHm-=dBjZYECGmfnydUicUyrQqndgX2MQ@mail.gmail.com>
@ 2022-02-24  9:22 ` Thorsten Leemhuis
  2022-04-11 10:17   ` Regression in workingset_refault latency on 5.15 #forregzbot Thorsten Leemhuis
  0 siblings, 1 reply; 3+ messages in thread
From: Thorsten Leemhuis @ 2022-02-24  9:22 UTC (permalink / raw)
  To: Daniel Dao, shakeelb
  Cc: kernel-team, linux-mm, hannes, guro, feng.tang, mhocko, hdanton,
	mkoutny, akpm, torvalds, regressions

[TLDR: I'm adding below regression report below to regzbot, the Linux
kernel regression tracking bot; all text you find below is compiled from
a few templates paragraphs you might have encountered already already
from similar mails.]

Hi, this is your Linux kernel regression tracker speaking.

Thanks for the report. CCing the regression mailing list, as it should
be in the loop for all regressions, as explained here:
https://www.kernel.org/doc/html/latest/admin-guide/reporting-issues.html

To be sure this issue doesn't fall through the cracks unnoticed, I'm
adding it to regzbot, my Linux kernel regression tracking bot:

#regzbot ^introduced v3.0..v5.15.19
#regzbot ignore-activity

Reminder for developers: when fixing the issue, please add a 'Link:'
tags pointing to the report (the mail quoted below) using
lore.kernel.org/r/, as explained in
'Documentation/process/submitting-patches.rst' and
'Documentation/process/5.Posting.rst'. This allows the bot to connect
the report with any patches posted or committed to fix the issue; this
again allows the bot to show the current status of regressions and
automatically resolve the issue when the fix hits the right tree.

I'm sending this to everyone that got the initial report, to make them
aware of the tracking. I also hope that messages like this motivate
people to directly get at least the regression mailing list and ideally
even regzbot involved when dealing with regressions, as messages like
this wouldn't be needed then.

Don't worry, I'll send further messages wrt to this regression just to
the lists (with a tag in the subject so people can filter them away), if
they are relevant just for regzbot. With a bit of luck no such messages
will be needed anyway.

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

P.S.: As the Linux kernel's regression tracker I'm getting a lot of
reports on my table. I can only look briefly into most of them and lack
knowledge about most of the areas they concern. I thus unfortunately
will sometimes get things wrong or miss something important. I hope
that's not the case here; if you think it is, don't hesitate to tell me
in a public reply, it's in everyone's interest to set the public record
straight.

On 23.02.22 14:51, Daniel Dao wrote:
> Hi all,
> 
> We are observing some regressions in workingset_refault on our newly upgraded
> 5.15.19 nodes with zram as swap. This manifests in several ways:
> 
> 1) Regression of workingset_refault duration observed in flamegraph
> 
> We regularly collect flamegraphs for running services on the node. Since upgrade
> to 5.15.19, we see that workingset_refault occupied a more significant part of
> the service flamegraph (13%) with the following call trace
> 
>   workingset_refault+0x128
>   add_to_page_cache_lru+0x9f
>   page_cache_ra_unbounded+0x154
>   force_page_cache_ra+0xe2
>   filemap_get_pages+0xe9
>   filemap_read+0xa4
>   xfs_file_buffered_read+0x98
>   xfs_file_read_iter+0x6a
>   new_sync_read+0x118
>   vfs_read+0xf2
>   __x64_sys_pread64+0x89
>   do_syscall_64+0x3b
>   entry_SYSCALL_64_after_hwframe+0x44
> 
> 2) Regression of userspace performance sensitive code
> 
> We have some performance sensentive code running in userspace that have their
> runtime measured by CLOCK_THREAD_CPUTIME_ID. They look roughly as:
> 
>   now = clock_gettime(CLOCK_THREAD_CPUTIME_ID)
>   func()
>   elapsed = clock_gettime(CLOCK_THREAD_CPUTIME_ID) - now
> 
> Since 5.15 upgrade, we observed long `elapsed` in the range of 4-10ms much more
> frequently than before.  This went away after we disabled swap for the service
> using `memory.swap.max=0` memcg configuration.
> 
> The new thing in 5.15 workingset_refault seems to be introduction of
> mem_cgroup_flush_stats()
> by commit 1f828223b7991a228bc2aef837b78737946d44b2 (memcg: flush
> lruvec stats in the
> refault).
> 
> Given that mem_cgroup_flush_stats can take quite a long time for us on the
> standard systemd cgroupv2 hierrachy ( root / system.slice / workload.service )
> 
>   sudo /usr/share/bcc/tools/funcslower -m 10 -t mem_cgroup_flush_stats
>   Tracing function calls slower than 10 ms... Ctrl+C to quit.
>   TIME       COMM           PID    LAT(ms)             RVAL FUNC
>   0.000000   <redacted>     804776   11.50              200
> mem_cgroup_flush_stats
>   0.343383   <redacted>     647496   10.58              200
> mem_cgroup_flush_stats
>   0.604309   <redacted>     804776   10.50              200
> mem_cgroup_flush_stats
>   1.230416   <redacted>     803293   10.01              200
> mem_cgroup_flush_stats
>   1.248442   <redacted>     646400   11.02              200
> mem_cgroup_flush_stats
> 
> could it be possible that workingset_refault in some unfortunate case can take
> much longer than before such that it increases the time observed by
> CLOCK_THREAD_CPUTIME_ID from userspace, or overall duration of
> workingset_refault
> observed by perf ?
> 

-- 
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