Re: [PATCH v3 1/1] mm: count time in drain_all_pages during direct reclaim as memory pressure

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Feb 24, 2022 at 12:53 AM 'Michal Hocko' via kernel-team
<kernel-team@android.com> wrote:
>
> On Wed 23-02-22 11:48:12, Suren Baghdasaryan wrote:
> > When page allocation in direct reclaim path fails, the system will
> > make one attempt to shrink per-cpu page lists and free pages from
> > high alloc reserves. Draining per-cpu pages into buddy allocator can
> > be a very slow operation because it's done using workqueues and the
> > task in direct reclaim waits for all of them to finish before
> > proceeding. Currently this time is not accounted as psi memory stall.
> >
> > While testing mobile devices under extreme memory pressure, when
> > allocations are failing during direct reclaim, we notices that psi
> > events which would be expected in such conditions were not triggered.
> > After profiling these cases it was determined that the reason for
> > missing psi events was that a big chunk of time spent in direct
> > reclaim is not accounted as memory stall, therefore psi would not
> > reach the levels at which an event is generated. Further investigation
> > revealed that the bulk of that unaccounted time was spent inside
> > drain_all_pages call.
> >
> > A typical captured case when drain_all_pages path gets activated:
> >
> > __alloc_pages_slowpath  took 44.644.613ns
> >     __perform_reclaim   took    751.668ns (1.7%)
> >     drain_all_pages     took 43.887.167ns (98.3%)
>
> Although the draining is done in the slow path these numbers suggest
> that we should really reconsider the use of WQ both for draining and
> other purposes (like vmstats).

Yep, I'm testing the kthread_create_worker_on_cpu approach suggested
by Petr. Will post it later today if nothing regresses.

>
> > PSI in this case records the time spent in __perform_reclaim but
> > ignores drain_all_pages, IOW it misses 98.3% of the time spent in
> > __alloc_pages_slowpath.
> >
> > Annotate __alloc_pages_direct_reclaim in its entirety so that delays
> > from handling page allocation failure in the direct reclaim path are
> > accounted as memory stall.
> >
> > Reported-by: Tim Murray <timmurray@google.com>
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > Acked-by: Johannes Weiner <hannes@cmpxchg.org>
>
> Acked-by: Michal Hocko <mhocko@suse.com>
>
> Thanks!
>
> > ---
> > changes in v3:
> > - Moved psi_memstall_leave after the "out" label
> >
> >  mm/page_alloc.c | 10 ++++++----
> >  1 file changed, 6 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 3589febc6d31..029bceb79861 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -4595,13 +4595,12 @@ __perform_reclaim(gfp_t gfp_mask, unsigned int order,
> >                                       const struct alloc_context *ac)
> >  {
> >       unsigned int noreclaim_flag;
> > -     unsigned long pflags, progress;
> > +     unsigned long progress;
> >
> >       cond_resched();
> >
> >       /* We now go into synchronous reclaim */
> >       cpuset_memory_pressure_bump();
> > -     psi_memstall_enter(&pflags);
> >       fs_reclaim_acquire(gfp_mask);
> >       noreclaim_flag = memalloc_noreclaim_save();
> >
> > @@ -4610,7 +4609,6 @@ __perform_reclaim(gfp_t gfp_mask, unsigned int order,
> >
> >       memalloc_noreclaim_restore(noreclaim_flag);
> >       fs_reclaim_release(gfp_mask);
> > -     psi_memstall_leave(&pflags);
> >
> >       cond_resched();
> >
> > @@ -4624,11 +4622,13 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
> >               unsigned long *did_some_progress)
> >  {
> >       struct page *page = NULL;
> > +     unsigned long pflags;
> >       bool drained = false;
> >
> > +     psi_memstall_enter(&pflags);
> >       *did_some_progress = __perform_reclaim(gfp_mask, order, ac);
> >       if (unlikely(!(*did_some_progress)))
> > -             return NULL;
> > +             goto out;
> >
> >  retry:
> >       page = get_page_from_freelist(gfp_mask, order, alloc_flags, ac);
> > @@ -4644,6 +4644,8 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
> >               drained = true;
> >               goto retry;
> >       }
> > +out:
> > +     psi_memstall_leave(&pflags);
> >
> >       return page;
> >  }
> > --
> > 2.35.1.473.g83b2b277ed-goog
>
> --
> Michal Hocko
> SUSE Labs
>
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