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

[Shakeel Butt]

On Fri, Jun 9, 2023 at 12:10 AM Dennis Zhou <dennisszhou@xxxxxxxxx> wrote:
>
> 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.
>

That would be awesome and thanks a lot.