linux-kernel - Re: [PATCH 1/4] memcg: do not try to drain per-cpu caches without pages

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Message-ID: <20110722095815.GF4004@tiehlicka.suse.cz>
Date:	Fri, 22 Jul 2011 11:58:16 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	linux-mm@...ck.org, Balbir Singh <bsingharora@...il.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] memcg: do not try to drain per-cpu caches without
 pages

On Fri 22-07-11 18:28:22, KAMEZAWA Hiroyuki wrote:
> On Fri, 22 Jul 2011 11:19:36 +0200
> Michal Hocko <mhocko@...e.cz> wrote:
> 
> > On Fri 22-07-11 08:44:13, KAMEZAWA Hiroyuki wrote:
> > > On Thu, 21 Jul 2011 13:36:06 +0200
> > > Michal Hocko <mhocko@...e.cz> wrote:
> > > 
> > > > On Thu 21-07-11 19:12:50, KAMEZAWA Hiroyuki wrote:
> > > > > On Thu, 21 Jul 2011 09:38:00 +0200
> > > > > Michal Hocko <mhocko@...e.cz> wrote:
> > > > > 
> > > > > > drain_all_stock_async tries to optimize a work to be done on the work
> > > > > > queue by excluding any work for the current CPU because it assumes that
> > > > > > the context we are called from already tried to charge from that cache
> > > > > > and it's failed so it must be empty already.
> > > > > > While the assumption is correct we can do it by checking the current
> > > > > > number of pages in the cache. This will also reduce a work on other CPUs
> > > > > > with an empty stock.
> > > > > > 
> > > > > > Signed-off-by: Michal Hocko <mhocko@...e.cz>
> > > > > 
> > > > > 
> > > > > At the first look, when a charge against TransParentHugepage() goes
> > > > > into the reclaim routine, stock->nr_pages != 0 and this will
> > > > > call additional kworker.
> > > > 
> > > > True. We will drain a charge which could be used by other allocations
> > > > in the meantime so we have a good chance to reclaim less. But how big
> > > > problem is that?
> > > > I mean I can add a new parameter that would force checking the current
> > > > cpu but it doesn't look nice. I cannot add that condition
> > > > unconditionally because the code will be shared with the sync path in
> > > > the next patch and that one needs to drain _all_ cpus.
> > > > 
> > > > What would you suggest?
> > > By 2 methods
> > > 
> > >  - just check nr_pages. 
> > 
> > Not sure I understand which nr_pages you mean. The one that comes from
> > the charging path or stock->nr_pages?
> > If you mean the first one then we do not have in the reclaim path where
> > we call drain_all_stock_async.
> > 
> 
> stock->nr_pages.
> 
> > >  - drain "local stock" without calling schedule_work(). It's fast.
> > 
> > but there is nothing to be drained locally in the paths where we call
> > drain_all_stock_async... Or do you mean that drain_all_stock shouldn't
> > use work queue at all?
> 
> I mean calling schedule_work against local cpu is just waste of time.
> Then, drain it directly and move local cpu's stock->nr_pages to res_counter.

got it. Thanks for clarification. Will repost the updated version.
 
> Thanks,
> -Kame

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