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

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Date:   Wed, 15 Nov 2017 09:56:02 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     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>,
        Shakeel Butt <shakeelb@...gle.com>,
        Greg Thelen <gthelen@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm,vmscan: Kill global shrinker lock.

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?

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