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Date:   Sun, 20 Feb 2022 08:52:38 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     akpm@...ux-foundation.org, hannes@...xchg.org, mhocko@...e.com,
        peterz@...radead.org, guro@...com, shakeelb@...gle.com,
        timmurray@...gle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH 1/1] mm: count time in drain_all_pages during direct
 reclaim as memory pressure

On Sat, Feb 19, 2022 at 4:40 PM Minchan Kim <minchan@...nel.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@...gle.com>
> > Signed-off-by: Suren Baghdasaryan <surenb@...gle.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.
Thanks,
Suren.

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