linux-kernel - Re: [PATCH 1/2] mm: Avoid full RCU lookup of memcg for statistics updates

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Message-ID: <20140723125818.GT10819@suse.de>
Date:	Wed, 23 Jul 2014 13:58:18 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 1/2] mm: Avoid full RCU lookup of memcg for statistics
 updates

On Wed, Jul 23, 2014 at 07:44:49AM -0400, Johannes Weiner wrote:
> On Wed, Jul 23, 2014 at 12:24:15PM +0100, Mel Gorman wrote:
> > When updating memcg VM statistics like PGFAULT we take the rcu read
> > lock and lookup the memcg. For statistic updates this is overkill
> > when the process may not belong to a memcg. This patch adds a light
> > check to check if a memcg potentially exists. It's race-prone in that
> > some VM stats may be missed when a process first joins a memcg but
> > that is not serious enough to justify a constant performance penalty.
> 
> Tasks always belong to a memcg, the root group per default.  There
> isn't really any accounting that could be omitted.
> 

Crap, ok, I had not taken that into account. The lookup of that cannot
really be avoided. It's a pity because the stats on the root memcg are
not likely to be that interesting. Thanks for reviewing.

-- 
Mel Gorman
SUSE Labs
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