From: Mel Gorman <mgorman@suse.de>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Michal Hocko <mhocko@suse.cz>,
Linux Kernel <linux-kernel@vger.kernel.org>,
Linux-MM <linux-mm@kvack.org>
Subject: Re: [PATCH 1/2] mm: Avoid full RCU lookup of memcg for statistics updates
Date: Wed, 23 Jul 2014 13:58:18 +0100 [thread overview]
Message-ID: <20140723125818.GT10819@suse.de> (raw)
In-Reply-To: <20140723114449.GE1725@cmpxchg.org>
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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WARNING: multiple messages have this Message-ID (diff)
From: Mel Gorman <mgorman@suse.de>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Michal Hocko <mhocko@suse.cz>,
Linux Kernel <linux-kernel@vger.kernel.org>,
Linux-MM <linux-mm@kvack.org>
Subject: Re: [PATCH 1/2] mm: Avoid full RCU lookup of memcg for statistics updates
Date: Wed, 23 Jul 2014 13:58:18 +0100 [thread overview]
Message-ID: <20140723125818.GT10819@suse.de> (raw)
In-Reply-To: <20140723114449.GE1725@cmpxchg.org>
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
next prev parent reply other threads:[~2014-07-23 12:58 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-07-23 11:24 [PATCH 0/2] Avoid unnecessary overhead in fault paths due to memcg and rss stats Mel Gorman
2014-07-23 11:24 ` Mel Gorman
2014-07-23 11:24 ` [PATCH 1/2] mm: Avoid full RCU lookup of memcg for statistics updates Mel Gorman
2014-07-23 11:24 ` Mel Gorman
2014-07-23 11:44 ` Johannes Weiner
2014-07-23 11:44 ` Johannes Weiner
2014-07-23 12:58 ` Mel Gorman [this message]
2014-07-23 12:58 ` Mel Gorman
2014-07-23 11:24 ` [PATCH 2/2] mm: Move RSS stat event count synchronisation out of the fast path Mel Gorman
2014-07-23 11:24 ` Mel Gorman
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