From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: Mel Gorman <mel@csn.ul.ie>
Subject: Re: [PATCH 1/3] Reintroduce zone_reclaim_interval for when zone_reclaim() scans and fails to avoid CPU spinning at 100% on NUMA
Cc: kosaki.motohiro@jp.fujitsu.com, Rik van Riel <riel@redhat.com>,
       Christoph Lameter <cl@linux-foundation.org>, yanmin.zhang@intel.com,
       Wu Fengguang <fengguang.wu@intel.com>, linuxram@us.ibm.com,
       linux-mm <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>
In-Reply-To: <20090609104404.GP18380@csn.ul.ie>
References: <20090609185036.DD8E.A69D9226@jp.fujitsu.com> <20090609104404.GP18380@csn.ul.ie>
Message-Id: <20090609194551.DD94.A69D9226@jp.fujitsu.com>
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Date: Tue,  9 Jun 2009 19:50:30 +0900 (JST)
Sender: linux-kernel-owner@vger.kernel.org
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> On Tue, Jun 09, 2009 at 06:59:03PM +0900, KOSAKI Motohiro wrote:
> > > > > > Here it is just recording the jiffies value. The real smarts with the counter
> > > > > > use time_before() which I assumed could handle jiffie wrap-arounds. Even
> > > > > > if it doesn't, the consequence is that one scan will occur that could have
> > > > > > been avoided around the time of the jiffie wraparound. The value will then
> > > > > > be reset and it will be fine.
> > > > > 
> > > > > time_before() assume two argument are enough nearly time.
> > > > > if we use 32bit cpu and HZ=1000, about jiffies wraparound about one month.
> > > > > 
> > > > > Then, 
> > > > > 
> > > > > 1. zone reclaim failure occur
> > > > > 2. system works fine for one month
> > > > > 3. jiffies wrap and time_before() makes mis-calculation.
> > > > > 
> > > > 
> > > > And the scan occurs uselessly and zone_reclaim_failure gets set again.
> > > > I believe the one useless scan is not significant enough to warrent dealing
> > > > with jiffie wraparound.
> > > 
> > > Thank you for kindful explanation.
> > > I fully agreed.
> > 
> > Bah, no, not agreed.
> > simple last failure recording makes following scenario.
> > 
> > 
> > 1. zone reclaim failure occur. update zone_reclaim_failure.
> >       ^
> >       |  time_before() return 1, and zone_reclaim() return immediately.
> >       v
> > 2. after 32 second.
> >       ^
> >       |  time_before() return 0, and zone_reclaim() works normally
> >       v
> > 3. after one month
> >       ^
> >       |  time_before() return 1, and zone_reclaim() return immediately.
> >       |  although recent zone_reclaim() never failed.
> >       v
> > 4. after more one month
> >       
> 
> Pants.
> 
> /me slaps self
> 
> +       /* Watch for jiffie wraparound */
> +       if (unlikely(jiffies < zone->zone_reclaim_failure))
> +               zone->zone_reclaim_failure = jiffies;
> +
> +       /* Do not attempt a scan if scanning failed recently */
> +       if (time_before(jiffies,
> +                       zone->zone_reclaim_failure + zone_reclaim_interval))
> +               return 0;
> +
> 
> ?

looks good.


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