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From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
To: mhocko@kernel.org
Cc: linux-mm@kvack.org, akpm@linux-foundation.org,
	hannes@cmpxchg.org, mgorman@suse.de,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: warn about allocations which stall for too long
Date: Tue, 27 Sep 2016 21:57:26 +0900	[thread overview]
Message-ID: <201609272157.DHI95301.HOFFFOVJLtSMQO@I-love.SAKURA.ne.jp> (raw)
In-Reply-To: <20160926081751.GD27030@dhcp22.suse.cz>

Michal Hocko wrote:
> > > > ) rather than by line number, and surround __warn_memalloc_stall() call with
> > > > mutex in order to serialize warning messages because it is possible that
> > > > multiple allocation requests are stalling?
> > > 
> > > we do not use any lock in warn_alloc_failed so why this should be any
> > > different?
> > 
> > warn_alloc_failed() is called for both __GFP_DIRECT_RECLAIM and
> > !__GFP_DIRECT_RECLAIM allocation requests, and it is not allowed
> > to sleep if !__GFP_DIRECT_RECLAIM. Thus, we have to tolerate that
> > concurrent memory allocation failure messages make dmesg output
> > unreadable. But __warn_memalloc_stall() is called for only
> > __GFP_DIRECT_RECLAIM allocation requests. Thus, we are allowed to
> > sleep in order to serialize concurrent memory allocation stall
> > messages.
> 
> I still do not see a point. A single line about the warning and locked
> dump_stack sounds sufficient to me.

printk() is slow operation. It is possible that two allocation requests
start within time period needed for completing warn_alloc_failed().
It is possible that multiple concurrent allocations are stalling when
one of them cannot be satisfied. The consequence is multiple concurrent
timeouts corrupting dmesg.
http://I-love.SAKURA.ne.jp/tmp/serial-20160927-nolock.txt.xz
(Please ignore Oops at do_task_stat(); it is irrelevant to this topic.)

If we guard it with mutex_lock(&oom_lock)/mutex_unlock(&oom_lock),
no corruption.
http://I-love.SAKURA.ne.jp/tmp/serial-20160927-lock.txt.xz

Deferring it when trylock() failed will be also possible.

WARNING: multiple messages have this Message-ID (diff)
From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
To: mhocko@kernel.org
Cc: linux-mm@kvack.org, akpm@linux-foundation.org,
	hannes@cmpxchg.org, mgorman@suse.de,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: warn about allocations which stall for too long
Date: Tue, 27 Sep 2016 21:57:26 +0900	[thread overview]
Message-ID: <201609272157.DHI95301.HOFFFOVJLtSMQO@I-love.SAKURA.ne.jp> (raw)
In-Reply-To: <20160926081751.GD27030@dhcp22.suse.cz>

Michal Hocko wrote:
> > > > ) rather than by line number, and surround __warn_memalloc_stall() call with
> > > > mutex in order to serialize warning messages because it is possible that
> > > > multiple allocation requests are stalling?
> > > 
> > > we do not use any lock in warn_alloc_failed so why this should be any
> > > different?
> > 
> > warn_alloc_failed() is called for both __GFP_DIRECT_RECLAIM and
> > !__GFP_DIRECT_RECLAIM allocation requests, and it is not allowed
> > to sleep if !__GFP_DIRECT_RECLAIM. Thus, we have to tolerate that
> > concurrent memory allocation failure messages make dmesg output
> > unreadable. But __warn_memalloc_stall() is called for only
> > __GFP_DIRECT_RECLAIM allocation requests. Thus, we are allowed to
> > sleep in order to serialize concurrent memory allocation stall
> > messages.
> 
> I still do not see a point. A single line about the warning and locked
> dump_stack sounds sufficient to me.

printk() is slow operation. It is possible that two allocation requests
start within time period needed for completing warn_alloc_failed().
It is possible that multiple concurrent allocations are stalling when
one of them cannot be satisfied. The consequence is multiple concurrent
timeouts corrupting dmesg.
http://I-love.SAKURA.ne.jp/tmp/serial-20160927-nolock.txt.xz
(Please ignore Oops at do_task_stat(); it is irrelevant to this topic.)

If we guard it with mutex_lock(&oom_lock)/mutex_unlock(&oom_lock),
no corruption.
http://I-love.SAKURA.ne.jp/tmp/serial-20160927-lock.txt.xz

Deferring it when trylock() failed will be also possible.

--
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  reply	other threads:[~2016-09-27 12:57 UTC|newest]

Thread overview: 42+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-09-23  8:15 [PATCH] mm: warn about allocations which stall for too long Michal Hocko
2016-09-23  8:15 ` Michal Hocko
2016-09-23  8:29 ` Hillf Danton
2016-09-23  8:29   ` Hillf Danton
2016-09-23  8:32   ` Michal Hocko
2016-09-23  8:32     ` Michal Hocko
2016-09-23  8:44     ` Hillf Danton
2016-09-23  8:44       ` Hillf Danton
2016-09-23  9:15       ` Michal Hocko
2016-09-23  9:15         ` Michal Hocko
2016-09-23 14:36 ` Tetsuo Handa
2016-09-23 14:36   ` Tetsuo Handa
2016-09-23 15:02   ` Michal Hocko
2016-09-23 15:02     ` Michal Hocko
2016-09-24  3:00     ` Tetsuo Handa
2016-09-24  3:00       ` Tetsuo Handa
2016-09-26  8:17       ` Michal Hocko
2016-09-26  8:17         ` Michal Hocko
2016-09-27 12:57         ` Tetsuo Handa [this message]
2016-09-27 12:57           ` Tetsuo Handa
2016-09-29  8:48           ` Michal Hocko
2016-09-29  8:48             ` Michal Hocko
2016-09-23 17:34 ` Dave Hansen
2016-09-23 17:34   ` Dave Hansen
2016-09-24 13:19   ` Balbir Singh
2016-09-24 13:19     ` Balbir Singh
2016-09-26  8:13     ` Michal Hocko
2016-09-26  8:13       ` Michal Hocko
2016-09-26  8:12   ` Michal Hocko
2016-09-26  8:12     ` Michal Hocko
2016-09-29  8:44 ` [PATCH 0/2] " Michal Hocko
2016-09-29  8:44   ` Michal Hocko
2016-09-29  8:44   ` [PATCH 1/2] mm: consolidate warn_alloc_failed users Michal Hocko
2016-09-29  8:44     ` Michal Hocko
2016-09-29  9:23     ` Vlastimil Babka
2016-09-29  9:23       ` Vlastimil Babka
2016-09-29  8:44   ` [PATCH 2/2] mm: warn about allocations which stall for too long Michal Hocko
2016-09-29  8:44     ` Michal Hocko
2016-09-29  9:02     ` Tetsuo Handa
2016-09-29  9:02       ` Tetsuo Handa
2016-09-29  9:10       ` Michal Hocko
2016-09-29  9:10         ` Michal Hocko

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