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References: <20230413214326.2147568-1-kaleshsingh@google.com>
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From:   Yu Zhao <yuzhao@google.com>
Date:   Thu, 13 Apr 2023 15:47:56 -0600
Message-ID: <CAOUHufYjLf2RD2Y7wcebjvvDRHDT9cCG6sPbQ2JaTyyB306JOw@mail.gmail.com>
Subject: Re: [PATCH] mm: Multi-gen LRU: remove wait_event_killable()
To:     Kalesh Singh <kaleshsingh@google.com>, akpm@linux-foundation.org
Cc:     minchan@google.com, surenb@google.com, wvw@google.com,
        android-mm@google.com, kernel-team@android.com,
        Minchan Kim <minchan@kernel.org>,
        Oleksandr Natalenko <oleksandr@natalenko.name>,
        "Jan Alexander Steffens (heftig)" <heftig@archlinux.org>,
        Suleiman Souhlal <suleiman@google.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
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On Thu, Apr 13, 2023 at 3:43=E2=80=AFPM Kalesh Singh <kaleshsingh@google.co=
m> wrote:
>
> Android 14 and later default to MGLRU [1] and field telemetry showed
> occasional long tail latency (>100ms) in the reclaim path.
>
> Tracing revealed priority inversion in the reclaim path. In
> try_to_inc_max_seq(), when high priority tasks were blocked on
> wait_event_killable(), the preemption of the low priority task to call
> wake_up_all() caused those high priority tasks to wait longer than
> necessary. In general, this problem is not different from others of
> its kind, e.g., one caused by mutex_lock(). However, it is specific to
> MGLRU because it introduced the new wait queue lruvec->mm_state.wait.
>
> The purpose of this new wait queue is to avoid the thundering herd
> problem. If many direct reclaimers rush into try_to_inc_max_seq(),
> only one can succeed, i.e., the one to wake up the rest, and the rest
> who failed might cause premature OOM kills if they do not wait. So far
> there is no evidence supporting this scenario, based on how often the
> wait has been hit. And this begs the question how useful the wait
> queue is in practice.
>
> Based on Minchan's recommendation, which is in line with his commit
> 6d4675e60135 ("mm: don't be stuck to rmap lock on reclaim path") and
> the rest of the MGLRU code which also uses trylock when possible,
> remove the wait queue.
>
> [1] https://android-review.googlesource.com/q/I7ed7fbfd6ef9ce100533475281=
25dd98c39e50bf
>
> Fixes: bd74fdaea146 ("mm: multi-gen LRU: support page table walks")
> Cc: Yu Zhao <yuzhao@google.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Reported-by: Wei Wang <wvw@google.com>
> Suggested-by: Minchan Kim <minchan@kernel.org>
> Signed-off-by: Kalesh Singh <kaleshsingh@google.com>

Acked-by: Yu Zhao <yuzhao@google.com>