From: Johannes Weiner <hannes@cmpxchg.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
Minchan Kim <minchan@kernel.org>,
Huang Ying <ying.huang@intel.com>,
Mel Gorman <mgorman@techsingularity.net>,
Vladimir Davydov <vdavydov.dev@gmail.com>,
Andrew Morton <akpm@linux-foundation.org>,
Shakeel Butt <shakeelb@google.com>,
Greg Thelen <gthelen@google.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] mm,vmscan: Kill global shrinker lock.
Date: Wed, 15 Nov 2017 09:00:20 -0500 [thread overview]
Message-ID: <20171115140020.GA6771@cmpxchg.org> (raw)
In-Reply-To: <20171115090251.umpd53zpvp42xkvi@dhcp22.suse.cz>
On Wed, Nov 15, 2017 at 10:02:51AM +0100, Michal Hocko wrote:
> On Tue 14-11-17 06:37:42, Tetsuo Handa wrote:
> > 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.
>
> I would use wait_event_interruptible in the remove path rather than the
> short sleep loop which is just too ugly. The shrinker walk would then
> just wake_up the sleeper when the ref. count drops to 0. Two
> synchronize_rcu is quite ugly as well, but I was not able to simplify
> them. I will keep thinking. It just sucks how we cannot follow the
> standard rcu list with dynamically allocated structure pattern here.
It's because the refcount is dropped too early. The refcount protects
the object during shrink, but not for the list_next(), and so you need
an additional grace period just for that part.
I think you could drop the reference count in the next iteration. This
way the list_next() works without requiring a second RCU grace period.
ref count protects the object and its list pointers; RCU protects what
the list pointers point to before we acquire the reference:
rcu_read_lock();
list_for_each_entry_rcu(pos, list) {
if (!atomic_inc_not_zero(&pos->ref))
continue;
rcu_read_unlock();
if (prev)
atomic_dec(&prev->ref);
prev = pos;
shrink();
rcu_read_lock();
}
rcu_read_unlock();
if (prev)
atomic_dec(&prev->ref);
In any case, Minchan's lock breaking seems way preferable over that
level of headscratching complexity for an unusual case like Shakeel's.
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next prev parent reply other threads:[~2017-11-15 14:00 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-11-13 21:37 [PATCH 1/2] mm,vmscan: Kill global shrinker lock Tetsuo Handa
2017-11-13 21:37 ` [PATCH 2/2] mm,vmscan: Allow parallel registration/unregistration of shrinkers Tetsuo Handa
2017-11-13 22:05 ` [PATCH 1/2] mm,vmscan: Kill global shrinker lock Shakeel Butt
2017-11-15 0:56 ` Minchan Kim
2017-11-15 6:28 ` Shakeel Butt
2017-11-16 0:46 ` Minchan Kim
2017-11-16 1:41 ` Shakeel Butt
2017-11-16 4:50 ` Minchan Kim
2017-11-15 8:56 ` Michal Hocko
2017-11-15 9:18 ` Michal Hocko
2017-11-16 17:44 ` Johannes Weiner
2017-11-23 23:46 ` Minchan Kim
2017-11-15 9:02 ` Michal Hocko
2017-11-15 10:58 ` Tetsuo Handa
2017-11-15 11:51 ` Michal Hocko
2017-11-16 0:56 ` Minchan Kim
2017-11-15 13:28 ` Johannes Weiner
2017-11-16 10:56 ` Tetsuo Handa
2017-11-15 14:00 ` Johannes Weiner [this message]
2017-11-15 14:11 ` Michal Hocko
2018-01-25 2:04 ` Tetsuo Handa
2018-01-25 8:36 ` Michal Hocko
2018-01-25 10:56 ` Tetsuo Handa
2018-01-25 11:41 ` Michal Hocko
2018-01-25 22:19 ` Eric Wheeler
2018-01-26 3:12 ` Tetsuo Handa
2018-01-26 10:08 ` Michal Hocko
2017-11-17 17:35 ` Christoph Hellwig
2017-11-17 17:41 ` Shakeel Butt
2017-11-17 17:53 ` Shakeel Butt
2017-11-17 18:36 ` Christoph Hellwig
2017-11-20 9:25 ` Michal Hocko
2017-11-20 9:33 ` Christoph Hellwig
2017-11-20 9:42 ` Michal Hocko
2017-11-20 10:41 ` Christoph Hellwig
2017-11-20 10:56 ` Tetsuo Handa
2017-11-20 18:28 ` Paul E. McKenney
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