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Date:   Wed, 23 Feb 2022 23:10:02 -0800
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Subject: Re: [PATCH v3 1/1] mm: count time in drain_all_pages during direct
 reclaim as memory pressure
From:   Shakeel Butt <shakeelb@google.com>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     akpm@linux-foundation.org, hannes@cmpxchg.org, mhocko@suse.com,
        pmladek@suse.com, peterz@infradead.org, guro@fb.com,
        minchan@kernel.org, timmurray@google.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kernel-team@android.com
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On Wed, Feb 23, 2022 at 11:48:12AM -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
> 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%)

> 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>

Reviewed-by: Shakeel Butt <shakeelb@google.com>