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Date:   Mon, 21 Feb 2022 09:55:12 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     akpm@linux-foundation.org, hannes@cmpxchg.org,
        peterz@infradead.org, guro@fb.com, shakeelb@google.com,
        minchan@kernel.org, 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
Message-ID: <YhNTcM9XtqA1zUUi@dhcp22.suse.cz>
References: <20220219174940.2570901-1-surenb@google.com>
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On Sat 19-02-22 09:49:40, 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.

It would be cool to have some numbers here.

> 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.

If the draining is too slow and dependent on the current CPU/WQ
contention then we should address that. The original intention was that
having a dedicated WQ with WQ_MEM_RECLAIM would help to isolate the
operation from the rest of WQ activity. Maybe we need to fine tune
mm_percpu_wq. If that doesn't help then we should revise the WQ model
and use something else. Memory reclaim shouldn't really get stuck behind
other unrelated work.
-- 
Michal Hocko
SUSE Labs