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

KAMEZAWA Hiroyuki
2011-06-15T01:19:02Z

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,
-kame