linux-kernel - Re: [patch 3/3] vmstat: Create our own workqueue

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Date:	Fri, 6 Nov 2015 20:28:52 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	cl@...ux.com
Cc:	htejun@...il.com, akpm@...ux-foundation.org, mhocko@...nel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, hannes@...xchg.org, mgorman@...e.de
Subject: Re: [patch 3/3] vmstat: Create our own workqueue

Christoph Lameter wrote:
> On Sat, 31 Oct 2015, Tetsuo Handa wrote:
> 
> > Then, you need to update below description (or drop it) because
> > patch 3/3 alone will not guarantee that the counters are up to date.
> 
> The vmstat system does not guarantee that the counters are up to date
> always. The whole point is the deferral of updates for performance
> reasons. They are updated *at some point* within stat_interval. That needs
> to happen and that is what this patchset is fixing.

So, if you refer to the blocking of the execution of vmstat updates,
description for patch 3/3 sould be updated to something like below?

----------
Since __GFP_WAIT memory allocations do not call schedule()
when there is nothing to reclaim, and workqueue does not kick
remaining workqueue items unless in-flight workqueue item calls
schedule(), __GFP_WAIT memory allocation requests by workqueue
items can block vmstat_update work item forever.

Since zone_reclaimable() decision depends on vmstat counters
to be up to dated, a silent lockup occurs because a workqueue
item doing a __GFP_WAIT memory allocation request continues
using outdated vmstat counters.

In order to fix this problem, we need to allocate a dedicated
workqueue for vmstat. Note that this patch itself does not fix
lockup problem. Tejun will develop a patch which detects lockup
situation and kick remaining workqueue items.
----------
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