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 8:28 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> 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.

An RFC for kthreads approach is posted at
https://lore.kernel.org/all/20220225012819.1807147-1-surenb@google.com/

>
> >
> > > 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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> >