From: Michal Hocko <mhocko@kernel.org> To: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> Cc: Minchan Kim <minchan@kernel.org>, Huang Ying <ying.huang@intel.com>, Mel Gorman <mgorman@techsingularity.net>, Vladimir Davydov <vdavydov.dev@gmail.com>, Johannes Weiner <hannes@cmpxchg.org>, 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 10:02:51 +0100 [thread overview] Message-ID: <20171115090251.umpd53zpvp42xkvi@dhcp22.suse.cz> (raw) In-Reply-To: <1510609063-3327-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp> On Tue 14-11-17 06:37:42, 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. Please provide some examples. I know your other patch mentions the usecase but I guess the two patches should be just squashed together. > 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. 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. > Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> > --- > include/linux/shrinker.h | 3 ++- > mm/vmscan.c | 41 +++++++++++++++++++---------------------- > 2 files changed, 21 insertions(+), 23 deletions(-) > > diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h > index 388ff29..333a1d0 100644 > --- a/include/linux/shrinker.h > +++ b/include/linux/shrinker.h > @@ -62,9 +62,10 @@ struct shrinker { > > int seeks; /* seeks to recreate an obj */ > long batch; /* reclaim batch size, 0 = default */ > - unsigned long flags; > + unsigned int flags; Why? > > /* These are for internal use */ > + atomic_t nr_active; > struct list_head list; > /* objs pending delete, per node */ > atomic_long_t *nr_deferred; > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 1c1bc95..c8996e8 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -157,7 +157,7 @@ struct scan_control { > unsigned long vm_total_pages; > > static LIST_HEAD(shrinker_list); > -static DECLARE_RWSEM(shrinker_rwsem); > +static DEFINE_MUTEX(shrinker_lock); > > #ifdef CONFIG_MEMCG > static bool global_reclaim(struct scan_control *sc) > @@ -285,9 +285,10 @@ int register_shrinker(struct shrinker *shrinker) > if (!shrinker->nr_deferred) > return -ENOMEM; > > - down_write(&shrinker_rwsem); > - list_add_tail(&shrinker->list, &shrinker_list); > - up_write(&shrinker_rwsem); > + atomic_set(&shrinker->nr_active, 0); I would expect ref counter to be 1 and either remove path dec it down to 0 or the racing walker would. In any case that is when unregister_shrinker can continue. -- Michal Hocko SUSE Labs
WARNING: multiple messages have this Message-ID (diff)
From: Michal Hocko <mhocko@kernel.org> To: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> Cc: Minchan Kim <minchan@kernel.org>, Huang Ying <ying.huang@intel.com>, Mel Gorman <mgorman@techsingularity.net>, Vladimir Davydov <vdavydov.dev@gmail.com>, Johannes Weiner <hannes@cmpxchg.org>, 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 10:02:51 +0100 [thread overview] Message-ID: <20171115090251.umpd53zpvp42xkvi@dhcp22.suse.cz> (raw) In-Reply-To: <1510609063-3327-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp> On Tue 14-11-17 06:37:42, 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. Please provide some examples. I know your other patch mentions the usecase but I guess the two patches should be just squashed together. > 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. 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. > Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> > --- > include/linux/shrinker.h | 3 ++- > mm/vmscan.c | 41 +++++++++++++++++++---------------------- > 2 files changed, 21 insertions(+), 23 deletions(-) > > diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h > index 388ff29..333a1d0 100644 > --- a/include/linux/shrinker.h > +++ b/include/linux/shrinker.h > @@ -62,9 +62,10 @@ struct shrinker { > > int seeks; /* seeks to recreate an obj */ > long batch; /* reclaim batch size, 0 = default */ > - unsigned long flags; > + unsigned int flags; Why? > > /* These are for internal use */ > + atomic_t nr_active; > struct list_head list; > /* objs pending delete, per node */ > atomic_long_t *nr_deferred; > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 1c1bc95..c8996e8 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -157,7 +157,7 @@ struct scan_control { > unsigned long vm_total_pages; > > static LIST_HEAD(shrinker_list); > -static DECLARE_RWSEM(shrinker_rwsem); > +static DEFINE_MUTEX(shrinker_lock); > > #ifdef CONFIG_MEMCG > static bool global_reclaim(struct scan_control *sc) > @@ -285,9 +285,10 @@ int register_shrinker(struct shrinker *shrinker) > if (!shrinker->nr_deferred) > return -ENOMEM; > > - down_write(&shrinker_rwsem); > - list_add_tail(&shrinker->list, &shrinker_list); > - up_write(&shrinker_rwsem); > + atomic_set(&shrinker->nr_active, 0); I would expect ref counter to be 1 and either remove path dec it down to 0 or the racing walker would. In any case that is when unregister_shrinker can continue. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2017-11-15 9:03 UTC|newest] Thread overview: 74+ 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 ` Tetsuo Handa 2017-11-13 21:37 ` [PATCH 2/2] mm,vmscan: Allow parallel registration/unregistration of shrinkers Tetsuo Handa 2017-11-13 21:37 ` Tetsuo Handa 2017-11-13 22:05 ` [PATCH 1/2] mm,vmscan: Kill global shrinker lock Shakeel Butt 2017-11-13 22:05 ` Shakeel Butt 2017-11-15 0:56 ` Minchan Kim 2017-11-15 0:56 ` Minchan Kim 2017-11-15 6:28 ` Shakeel Butt 2017-11-15 6:28 ` Shakeel Butt 2017-11-16 0:46 ` Minchan Kim 2017-11-16 0:46 ` Minchan Kim 2017-11-16 1:41 ` Shakeel Butt 2017-11-16 1:41 ` Shakeel Butt 2017-11-16 4:50 ` Minchan Kim 2017-11-16 4:50 ` Minchan Kim 2017-11-15 8:56 ` Michal Hocko 2017-11-15 8:56 ` Michal Hocko 2017-11-15 9:18 ` Michal Hocko 2017-11-15 9:18 ` Michal Hocko 2017-11-16 17:44 ` Johannes Weiner 2017-11-16 17:44 ` Johannes Weiner 2017-11-23 23:46 ` Minchan Kim 2017-11-23 23:46 ` Minchan Kim 2017-11-15 9:02 ` Michal Hocko [this message] 2017-11-15 9:02 ` Michal Hocko 2017-11-15 10:58 ` Tetsuo Handa 2017-11-15 10:58 ` Tetsuo Handa 2017-11-15 11:51 ` Michal Hocko 2017-11-15 11:51 ` Michal Hocko 2017-11-16 0:56 ` Minchan Kim 2017-11-16 0:56 ` Minchan Kim 2017-11-15 13:28 ` Johannes Weiner 2017-11-15 13:28 ` Johannes Weiner 2017-11-16 10:56 ` Tetsuo Handa 2017-11-16 10:56 ` Tetsuo Handa 2017-11-15 14:00 ` Johannes Weiner 2017-11-15 14:00 ` Johannes Weiner 2017-11-15 14:11 ` Michal Hocko 2017-11-15 14:11 ` Michal Hocko 2018-01-25 2:04 ` Tetsuo Handa 2018-01-25 2:04 ` Tetsuo Handa 2018-01-25 8:36 ` Michal Hocko 2018-01-25 8:36 ` Michal Hocko 2018-01-25 10:56 ` Tetsuo Handa 2018-01-25 10:56 ` Tetsuo Handa 2018-01-25 11:41 ` Michal Hocko 2018-01-25 11:41 ` Michal Hocko 2018-01-25 22:19 ` Eric Wheeler 2018-01-25 22:19 ` Eric Wheeler 2018-01-26 3:12 ` Tetsuo Handa 2018-01-26 3:12 ` Tetsuo Handa 2018-01-26 10:08 ` Michal Hocko 2018-01-26 10:08 ` Michal Hocko 2017-11-17 17:35 ` Christoph Hellwig 2017-11-17 17:35 ` Christoph Hellwig 2017-11-17 17:41 ` Shakeel Butt 2017-11-17 17:41 ` Shakeel Butt 2017-11-17 17:53 ` Shakeel Butt 2017-11-17 17:53 ` Shakeel Butt 2017-11-17 18:36 ` Christoph Hellwig 2017-11-17 18:36 ` Christoph Hellwig 2017-11-20 9:25 ` Michal Hocko 2017-11-20 9:25 ` Michal Hocko 2017-11-20 9:33 ` Christoph Hellwig 2017-11-20 9:33 ` Christoph Hellwig 2017-11-20 9:42 ` Michal Hocko 2017-11-20 9:42 ` Michal Hocko 2017-11-20 10:41 ` Christoph Hellwig 2017-11-20 10:41 ` Christoph Hellwig 2017-11-20 10:56 ` Tetsuo Handa 2017-11-20 10:56 ` Tetsuo Handa 2017-11-20 18:28 ` Paul E. McKenney 2017-11-20 18:28 ` Paul E. McKenney
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