linux-kernel - Re: [PATCH 1/2] mm,vmscan: Kill global shrinker lock.

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Date:   Tue, 14 Nov 2017 22:28:10 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Huang Ying <ying.huang@...el.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Thelen <gthelen@...gle.com>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm,vmscan: Kill global shrinker lock.

On Tue, Nov 14, 2017 at 4:56 PM, Minchan Kim <minchan@...nel.org> wrote:
> On Tue, Nov 14, 2017 at 06:37:42AM +0900, Tetsuo Handa wrote:
>> When shrinker_rwsem was introduced, it was assumed that
>> register_shrinker()/unregister_shrinker() are really unlikely paths
>> which are called during initialization and tear down. But nowadays,
>> register_shrinker()/unregister_shrinker() might be called regularly.
>> This patch prepares for allowing parallel registration/unregistration
>> of shrinkers.
>>
>> Since do_shrink_slab() can reschedule, we cannot protect shrinker_list
>> using one RCU section. But using atomic_inc()/atomic_dec() for each
>> do_shrink_slab() call will not impact so much.
>>
>> This patch uses polling loop with short sleep for unregister_shrinker()
>> rather than wait_on_atomic_t(), for we can save reader's cost (plain
>> atomic_dec() compared to atomic_dec_and_test()), we can expect that
>> do_shrink_slab() of unregistering shrinker likely returns shortly, and
>> we can avoid khungtaskd warnings when do_shrink_slab() of unregistering
>> shrinker unexpectedly took so long.
>>
>> Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
>
> Before reviewing this patch, can't we solve the problem with more
> simple way? Like this.
>
> Shakeel, What do you think?
>

Seems simple enough. I will run my test (running fork bomb in one
memcg and separately time a mount operation) and update if numbers
differ significantly.

> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 13d711dd8776..cbb624cb9baa 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -498,6 +498,14 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>                         sc.nid = 0;
>
>                 freed += do_shrink_slab(&sc, shrinker, nr_scanned, nr_eligible);
> +               /*
> +                * bail out if someone want to register a new shrinker to prevent
> +                * long time stall by parallel ongoing shrinking.
> +                */
> +               if (rwsem_is_contended(&shrinker_rwsem)) {
> +                       freed = 1;

freed = freed ?: 1;

> +                       break;
> +               }
>         }
>
>         up_read(&shrinker_rwsem);