[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 ? > -- Additional information about regzbot: If you want to know more about regzbot, check out its web-interface, the getting start guide, and the references documentation: https://linux-regtracking.leemhuis.info/regzbot/ https://gitlab.com/knurd42/regzbot/-/blob/main/docs/getting_started.md https://gitlab.com/knurd42/regzbot/-/blob/main/docs/reference.md The last two documents will explain how you can interact with regzbot yourself if your want to. Hint for reporters: when reporting a regression it's in your interest to CC the regression list and tell regzbot about the issue, as that ensures the regression makes it onto the radar of the Linux kernel's regression tracker -- that's in your interest, as it ensures your report won't fall through the cracks unnoticed. Hint for developers: you normally don't need to care about regzbot once it's involved. Fix the issue as you normally would, just remember to include 'Link:' tag in the patch descriptions pointing to all reports about the issue. This has been expected from developers even before regzbot showed up for reasons explained in 'Documentation/process/submitting-patches.rst' and 'Documentation/process/5.Posting.rst'.
