linux-kernel - Re: [patch 06/12] mm: oom_kill: simplify OOM killer locking

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Date:	Thu, 26 Mar 2015 14:31:11 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Huang Ying <ying.huang@...el.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Dave Chinner <david@...morbit.com>,
	Theodore Ts'o <tytso@....edu>
Subject: Re: [patch 06/12] mm: oom_kill: simplify OOM killer locking

On Wed 25-03-15 02:17:10, Johannes Weiner wrote:
> The zonelist locking and the oom_sem are two overlapping locks that
> are used to serialize global OOM killing against different things.
> 
> The historical zonelist locking serializes OOM kills from allocations
> with overlapping zonelists against each other to prevent killing more
> tasks than necessary in the same memory domain.  Only when neither
> tasklists nor zonelists from two concurrent OOM kills overlap (tasks
> in separate memcgs bound to separate nodes) are OOM kills allowed to
> execute in parallel.
> 
> The younger oom_sem is a read-write lock to serialize OOM killing
> against the PM code trying to disable the OOM killer altogether.
> 
> However, the OOM killer is a fairly cold error path, there is really
> no reason to optimize for highly performant and concurrent OOM kills.
> And the oom_sem is just flat-out redundant.
> 
> Replace both locking schemes with a single global mutex serializing
> OOM kills regardless of context.

OK, this is much simpler.

You have missed drivers/tty/sysrq.c which should take the lock as well.
ZONE_OOM_LOCKED can be removed as well. __out_of_memory in the kerneldoc
should be renamed.

[...]
> @@ -795,27 +728,21 @@ bool out_of_memory(struct zonelist *zonelist, gfp_t gfp_mask,
>   */
>  void pagefault_out_of_memory(void)
>  {
> -	struct zonelist *zonelist;
> -
> -	down_read(&oom_sem);
>  	if (mem_cgroup_oom_synchronize(true))
> -		goto unlock;
> +		return;

OK, so we are back to what David has asked previously. We do not need
the lock for memcg and oom_killer_disabled because we know that no tasks
(except for potential oom victim) are lurking around at the time
oom_killer_disable() is called. So I guess we want to stick a comment
into mem_cgroup_oom_synchronize before we check for oom_killer_disabled.

After those are fixed, feel free to add
Acked-by: Michal Hocko <mhocko@...e.cz>

-- 
Michal Hocko
SUSE Labs
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