linux-kernel - Re: [PATCH -mm] throttle direct reclaim when too many pages are isolated already

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Message-Id: <20090715202114.789d36f7.akpm@linux-foundation.org>
Date:	Wed, 15 Jul 2009 20:21:14 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: [PATCH -mm] throttle direct reclaim when too many pages are
 isolated already

On Wed, 15 Jul 2009 23:10:43 -0400 Rik van Riel <riel@...hat.com> wrote:

> Andrew Morton wrote:
> > On Wed, 15 Jul 2009 22:38:53 -0400 Rik van Riel <riel@...hat.com> wrote:
> > 
> >> When way too many processes go into direct reclaim, it is possible
> >> for all of the pages to be taken off the LRU.  One result of this
> >> is that the next process in the page reclaim code thinks there are
> >> no reclaimable pages left and triggers an out of memory kill.
> >>
> >> One solution to this problem is to never let so many processes into
> >> the page reclaim path that the entire LRU is emptied.  Limiting the
> >> system to only having half of each inactive list isolated for
> >> reclaim should be safe.
> >>
> > 
> > Since when?  Linux page reclaim has a bilion machine years testing and
> > now stuff like this turns up.  Did we break it or is this a
> > never-before-discovered workload?
> 
> It's been there for years, in various forms.  It hardly ever
> shows up, but Kosaki's patch series give us a nice chance to
> fix it for good.

OK.

> >> @@ -1049,6 +1070,10 @@ static unsigned long shrink_inactive_lis
> >>  	struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(zone, sc);
> >>  	int lumpy_reclaim = 0;
> >>  
> >> +	while (unlikely(too_many_isolated(zone, file))) {
> >> +		schedule_timeout_interruptible(HZ/10);
> >> +	}
> > 
> > This (incorrectly-laid-out) code is a no-op if signal_pending().
> 
> Good point, I should add some code to break out of page reclaim
> if a fatal signal is pending,

We can't just return NULL from __alloc_pages(), and if we can't
get a page from the freelists then we're just going to have to keep
reclaiming.  So I'm not sure how we can do this.

> and use a normal schedule_timeout
> otherwise.

congestion_wait() would be typical.

> Btw, how is this laid out wrong?  How do I do this better?

ask checkpatch ;)

WARNING: braces {} are not necessary for single statement blocks
#99: FILE: mm/vmscan.c:1073:
+	while (unlikely(too_many_isolated(zone, file))) {
+		schedule_timeout_interruptible(HZ/10);
+	}

total: 0 errors, 1 warnings, 37 lines checked

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