linux-kernel - Re: [PATCH] mm: Fix kswapd livelock on single core, no preempt kernel

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Date:	Wed, 14 Dec 2011 10:24:50 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Mike Waychison <mikew@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <jweiner@...hat.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Hugh Dickens <hughd@...gle.com>,
	Greg Thelen <gthelen@...gle.com>
Subject: Re: [PATCH] mm: Fix kswapd livelock on single core, no preempt
 kernel

On Wed, 2011-12-14 at 01:44 +0800, Mike Waychison wrote:
> On a single core system with kernel preemption disabled, it is possible
> for the memory system to be so taxed that kswapd cannot make any forward
> progress.  This can happen when most of system memory is tied up as
> anonymous memory without swap enabled, causing kswapd to consistently
> fail to achieve its watermark goals.  In turn, sleeping_prematurely()
> will consistently return true and kswapd_try_to_sleep() to never invoke
> schedule().  This causes the kswapd thread to stay on the CPU in
> perpetuity and keeps other threads from processing oom-kills to reclaim
> memory.
> 
> The cond_resched() instance in balance_pgdat() is never called as the
> loop that iterates from DEF_PRIORITY down to 0 will always set
> all_zones_ok to true, and not set it to false once we've passed
> DEF_PRIORITY as zones that are marked ->all_unreclaimable are not
> considered in the "all_zones_ok" evaluation.
> 
> This change modifies kswapd_try_to_sleep to ensure that we enter
> scheduler at least once per invocation if needed.  This allows kswapd to
> get off the CPU and allows other threads to die off from the OOM killer
> (freeing memory that is otherwise unavailable in the process).
your description suggests zones with all_unreclaimable set. but in this
case sleeping_prematurely() will return false instead of true, kswapd
will do sleep then. is there anything I missed?

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