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

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Message-Id: <20110722182822.a99a2676.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Fri, 22 Jul 2011 18:28:22 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Michal Hocko <mhocko@...e.cz>
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 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.

Thanks,
-Kame




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