linux-kernel - Re: [PATCH] swap: remove the struct cpumask has

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Date:	Thu, 31 Jul 2014 12:09:19 -0400
From:	Chris Metcalf <cmetcalf@...era.com>
To:	Michal Hocko <mhocko@...e.cz>, Lai Jiangshan <laijs@...fujitsu.com>
CC:	<linux-kernel@...r.kernel.org>, <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Tejun Heo <tj@...nel.org>,
	Christoph Lameter <cl@...two.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Jianyu Zhan <nasa4836@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Khalid Aziz <khalid.aziz@...cle.com>, <linux-mm@...ck.org>,
	Gilad Ben-Yossef <gilad@...yossef.com>
Subject: Re: [PATCH] swap: remove the struct cpumask has_work

On 7/31/2014 7:51 AM, Michal Hocko wrote:
> On Thu 31-07-14 11:30:19, Lai Jiangshan wrote:
>> It is suggested that cpumask_var_t and alloc_cpumask_var() should be used
>> instead of struct cpumask.  But I don't want to add this complicity nor
>> leave this unwelcome "static struct cpumask has_work;", so I just remove
>> it and use flush_work() to perform on all online drain_work.  flush_work()
>> performs very quickly on initialized but unused work item, thus we don't
>> need the struct cpumask has_work for performance.
> Why? Just because there is general recommendation for using
> cpumask_var_t rather than cpumask?
>
> In this particular case cpumask shouldn't matter much as it is static.
> Your code will work as well, but I do not see any strong reason to
> change it just to get rid of cpumask which is not on stack.

The code uses for_each_cpu with a cpumask to avoid waking cpus that don't
need to do work.  This is important for the nohz_full type functionality,
power efficiency, etc.  So, nack for this change.

-- 
Chris Metcalf, Tilera Corp.
http://www.tilera.com

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