Re: [BUGFIX][PATCH 5/5] memcg: fix percpu cached charge draining frequency

Michal Hocko
2011-06-15T07:15:06Z

On Wed 15-06-11 10:12:02, KAMEZAWA Hiroyuki wrote:
> On Wed, 15 Jun 2011 09:12:45 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
>
> > On Tue, 14 Jun 2011 09:36:51 +0200
> > Michal Hocko <mhocko@suse.cz> wrote:
> >
> > > On Mon 13-06-11 12:16:48, KAMEZAWA Hiroyuki wrote:
> > > > From 18b12e53f1cdf6d7feed1f9226c189c34866338c Mon Sep 17 00:00:00 2001
> > > > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > > > Date: Mon, 13 Jun 2011 11:25:43 +0900
> > > > Subject: [PATCH 5/5] memcg: fix percpu cached charge draining frequency
> > > >
> > > > For performance, memory cgroup caches some "charge" from res_counter
> > > > into per cpu cache. This works well but because it's cache,
> > > > it needs to be flushed in some cases. Typical cases are
> > > > 1. when someone hit limit.
> > > > 2. when rmdir() is called and need to charges to be 0.
> > > >
> > > > But "1" has problem.
> > > >
> > > > Recently, with large SMP machines, we see many kworker runs because
> > > > of flushing memcg's cache. Bad things in implementation are
> > > > that even if a cpu contains a cache for memcg not related to
> > > > a memcg which hits limit, drain code is called.
> > > >
> > > > This patch does
> > > > D) don't call at softlimit reclaim.
> > >
> > > I think this needs some justification. The decision is not that
> > > obvious IMO. I would say that this is a good decision because cached
> > > charges will not help to free any memory (at least not directly) during
> > > background reclaim. What about something like:
> > > "
> > > We are not draining per cpu cached charges during soft limit reclaim
> > > because background reclaim doesn't care about charges. It tries to free
> > > some memory and charges will not give any.
> > > Cached charges might influence only selection of the biggest soft limit
> > > offender but as the call is done only after the selection has been
> > > already done it makes no change.
> > > "
> > >
> > > Anyway, wouldn't it be better to have this change separate from the
> > > async draining logic change?
> >
> > Hmm. I think calling "draining" at softlimit is just a bug.
> >
> I'll divide patches.

Thanks.

--
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9
Czech Republic