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

[Dennis Zhou]

Hi Shakeel and Jan,

On Thu, Jun 08, 2023 at 05:37:00PM +0000, Shakeel Butt wrote:
> On Thu, Jun 08, 2023 at 01:14:08PM +0200, Jan Kara wrote:
> [...]
> > 
> > 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%:
> > 
> >                        before (9cd6ffa6025)  after (f1a7941243c1)
> > Amean     User         471.12 *   0.30%*     481.77 *  -1.96%*
> > Amean     System       244.47 *   0.90%*     269.13 *  -9.09%*
> > Amean     Elapsed      709.22 *   0.45%*     742.27 *  -4.19%*
> > Amean     CPU          100.00 (   0.20%)     101.00 *  -0.80%*
> > 
> > Essentially this workload spawns in sequence a lot of short-lived tasks and
> > the task startup + teardown cost is what this patch increases. To
> > demonstrate this more clearly, I've written trivial (and somewhat stupid)
> > benchmark shell_bench.sh:
> > 
> > for (( i = 0; i < 20000; i++ )); do
> > 	/bin/true
> > done
> > 
> > And when run like:
> > 
> > numactl -C 1 ./shell_bench.sh
> > 
> > (I've forced physical CPU binding to avoid task migrating over the machine
> > and cpu frequency scaling interfering which makes the numbers much more
> > noisy) I get the following elapsed times:
> > 
> >          9cd6ffa6025    f1a7941243c1
> > Avg      6.807429       7.631571
> > Stddev   0.021797       0.016483
> > 
> > So some 12% regression in elapsed time. Just to be sure I've verified that
> > per-cpu allocator patch [1] does not improve these numbers in any
> > significant way.
> > 
> > Where do we go from here? I think in principle the problem could be fixed
> > by being clever and when the task has only a single thread, we don't bother
> > with allocating pcpu counter (and summing it at the end) and just account
> > directly in mm_struct. When the second thread is spawned, we bite the
> > bullet, allocate pcpu counter and start with more scalable accounting.
> > These shortlived tasks in shell workloads or similar don't spawn any
> > threads so this should fix the regression. But this is obviously easier
> > said than done...
> > 
> 
> Thanks Jan for the report. I wanted to improve the percpu allocation to
> eliminate this regression as it was reported by intel test bot as well.
> However your suggestion seems seems targetted and reasonable as well. At
> the moment I am travelling, so not sure when I will get to this. Do you
> want to take a stab at it or you want me to do it? Also how urgent and
> sensitive this regression is for you?
> 
> thanks,
> Shakeel
> 
> 

I _think_ I could probably spin you a percpu_alloc_bulk() series in a
couple days for percpu_counters. Let me try and find some time, unless
you had something different in mind.

Thanks,
Dennis