On Thu, Dec 09, 2021 at 05:22:05PM +0000, Valentin Schneider wrote:

On 06/12/21 09:48, Valentin Schneider wrote:

quoted

On 03/12/21 14:00, Josef Bacik wrote:

quoted

On Fri, Dec 03, 2021 at 12:03:27PM +0000, Valentin Schneider wrote:

quoted

Could you give the 4 top patches, i.e. those above
8c92606ab810 ("sched/cpuacct: Make user/system times in cpuacct.stat more precise")
a try?

https://git.gitlab.arm.com/linux-arm/linux-vs.git -b mainline/sched/nohz-next-update-regression

I gave that a quick test on the platform that caused me to write the patch
you bisected and looks like it didn't break the original fix. If the above
counter-measures aren't sufficient, I'll have to go poke at your
reproducers...

It's better but still around 6% regression.  If I compare these patches to the
average of the last few days worth of runs you're 5% better than before, so
progress but not completely erased.

Hmph, time for me to reproduce this locally then. Thanks!

I carved out a partition out of an Ampere eMAG's HDD to play with BTRFS
via fsperf; this is what I get for the bisected commit (baseline is
bisected patchset's immediate parent, aka v5.15-rc4) via a handful of
./fsperf -p before-regression -c btrfs -n 100 -t emptyfiles500k

  write_clat_ns_p99     195395.92     198790.46      4797.01    1.74%
  write_iops             17305.79      17471.57       250.66    0.96%

  write_clat_ns_p99     195395.92     197694.06      4797.01    1.18%
  write_iops             17305.79      17533.62       250.66    1.32%

  write_clat_ns_p99     195395.92     197903.67      4797.01    1.28%
  write_iops             17305.79      17519.71       250.66    1.24%

If I compare against tip/sched/core however:

  write_clat_ns_p99     195395.92     202936.32      4797.01    3.86%
  write_iops             17305.79      17065.46       250.66   -1.39%

  write_clat_ns_p99     195395.92     204349.44      4797.01    4.58%
  write_iops             17305.79      17097.79       250.66   -1.20%

  write_clat_ns_p99     195395.92     204169.05      4797.01    4.49%
  write_iops             17305.79      17112.29       250.66   -1.12%

tip/sched/core + my patches:

  write_clat_ns_p99     195395.92     205721.60      4797.01    5.28%
  write_iops             17305.79      16947.59       250.66   -2.07%

  write_clat_ns_p99     195395.92     203358.04      4797.01    4.07%
  write_iops             17305.79      16953.24       250.66   -2.04%

  write_clat_ns_p99     195395.92     201830.40      4797.01    3.29%
  write_iops             17305.79      17041.18       250.66   -1.53%

So tip/sched/core seems to have a much worse regression, and my patches
are making things worse on that system...

I've started a bisection to see where the above leads me, unfortunately
this machine needs more babysitting than I thought so it's gonna take a
while.

@Josef any chance you could see if the above also applies to you? tip lives
at https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git, though from
where my bisection is taking me it looks like you should see that against
Linus' tree as well.

This has made us all curious, so we're all fucking around with schbench to see
if we can make it show up without needing to use fsperf.  Maybe that'll help
with the bisect, because I had to bisect twice to land on your patches, and I
only emailed when I could see the change right before and right after your
patch.  It would not surprise me at all if there's something else here that's
causing us pain.

Thanks,
Valentin