linux-kernel - Re: [PATCH] mm: call cond_resched() per MAX_ORDER_NR

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Message-ID: <528A94C4.80101@sr71.net>
Date:	Mon, 18 Nov 2013 14:29:24 -0800
From:	Dave Hansen <dave@...1.net>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
CC:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	dave.jiang@...el.com, akpm@...ux-foundation.org, dhillf@...il.com,
	Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH] mm: call cond_resched() per MAX_ORDER_NR_PAGES pages
 copy

On 11/18/2013 01:56 PM, Naoya Horiguchi wrote:
>> > Why bother trying to "optimize" it?
> I thought that if we call cond_resched() too often, the copying thread can
> take too long in a heavy load system, because the copying thread always
> yields the CPU in every loop.

I think you're confusing cond_resched() and yield().  The way I look at it:

yield() means: "Hey scheduler, go right now and run something else I'm
done running"

cond_resched() means: "Schedule me off if the scheduler has already
decided something else _should_ be running"

I'm sure I'm missing some of the subtleties, but as I see it, yield()
actively goes off and finds something else to run.  cond_resched() only
schedules you off if you've *already* run too long.


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