linux-kernel - Re: [PATCH 1/3] Reintroduce zone_reclaim_interval for when zone_reclaim() scans and fails to avoid CPU spinning at 100% on NUMA

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Message-Id: <20090609185036.DD8E.A69D9226@jp.fujitsu.com>
Date:	Tue,  9 Jun 2009 18:59:03 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	kosaki.motohiro@...fujitsu.com, Mel Gorman <mel@....ul.ie>,
	Rik van Riel <riel@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	yanmin.zhang@...el.com, Wu Fengguang <fengguang.wu@...el.com>,
	linuxram@...ibm.com, linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] Reintroduce zone_reclaim_interval for when zone_reclaim() scans and fails to avoid CPU spinning at 100% on NUMA

> > > > 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
      



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