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Date:   Wed, 23 Feb 2022 13:54:56 -0500
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     Minchan Kim <minchan@kernel.org>, akpm@linux-foundation.org,
        mhocko@suse.com, peterz@infradead.org, guro@fb.com,
        shakeelb@google.com, timmurray@google.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kernel-team@android.com
Subject: Re: [PATCH 1/1] mm: count time in drain_all_pages during direct
 reclaim as memory pressure
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On Sun, Feb 20, 2022 at 08:52:38AM -0800, Suren Baghdasaryan wrote:
> On Sat, Feb 19, 2022 at 4:40 PM Minchan Kim <minchan@kernel.org> wrote:
> >
> > On Sat, Feb 19, 2022 at 09:49:40AM -0800, 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
> >
> > Yes, drain_all_pages is serious slow(100ms - 150ms on Android)
> > especially when CPUs are fully packed. It was also spotted in CMA
> > allocation even when there was on no memory pressure.
> 
> Thanks for the input, Minchan!
> In my tests I've seen 50-60ms delays in a single drain_all_pages but I
> can imagine there are cases worse than these.
> 
> >
> > > proceeding. Currently this time is not accounted as psi memory stall.
> >
> > Good spot.
> >
> > >
> > > 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.
> > >
> > > Annotate drain_all_pages and unreserve_highatomic_pageblock during
> > > page allocation failure in the direct reclaim path so that delays
> > > caused by these calls are accounted as memory stall.
> > >
> > > Reported-by: Tim Murray <timmurray@google.com>
> > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > > ---
> > >  mm/page_alloc.c | 4 ++++
> > >  1 file changed, 4 insertions(+)
> > >
> > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > index 3589febc6d31..7fd0d392b39b 100644
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -4639,8 +4639,12 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
> > >        * Shrink them and try again
> > >        */
> > >       if (!page && !drained) {
> > > +             unsigned long pflags;
> > > +
> > > +             psi_memstall_enter(&pflags);
> > >               unreserve_highatomic_pageblock(ac, false);
> > >               drain_all_pages(NULL);
> > > +             psi_memstall_leave(&pflags);
> >
> > Instead of annotating the specific drain_all_pages, how about
> > moving the annotation from __perform_reclaim to
> > __alloc_pages_direct_reclaim?
> 
> I'm fine with that approach too. Let's wait for Johannes' input before
> I make any changes.

I think the change makes sense, even if the workqueue fix speeds up
the drain. I agree with Minchan about moving the annotation upward.

With it moved, please feel free to add
Acked-by: Johannes Weiner <hannes@cmpxchg.org>