linux-kernel - Re: [Patch] Call cond_resched() at bottom of main look in balance

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Message-Id: <20100628121056.408cbca2.akpm@linux-foundation.org>
Date:	Mon, 28 Jun 2010 12:10:56 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Johannes Weiner <hannes@...xchg.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Larry Woodman <lwoodman@...hat.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [Patch] Call cond_resched() at bottom of main look in 
 balance_pgdat()

On Wed, 23 Jun 2010 08:07:34 +0900
Minchan Kim <minchan.kim@...il.com> wrote:

> Acked-by: Johannes Weiner <hannes@...xchg.org>
> Reviewed-by: Minchan Kim <minchan.kim@...il.com>
> Reviewed-by: Rik van Riel <riel@...hat.com>
> Reviewed-by: KOSAKI Motohiro<kosaki.motohiro@...fujitsu.com>

The patch is a bit sucky, isn't it?

a) the cond_resched() which Larry's patch adds is very special.  It
   _looks_ like a random preemption point but it's actually critical to
   the correct functioning of the system.  That's utterly unobvious to
   anyone who reads the code, so a comment explaining this *must* be
   included.

b) cond_resched() is a really crappy way of solving the problem
   which Larry described.  It will sit there chewing away CPU time
   until kswapd's timeslice expires.

I suppose we can live with b) although it _does_ suck and I'd suggest
that the comment include a big FIXME, so someone might fix it.

Larry, please fix a), gather the acks and reviewed-by's, update the
changelog to identify the commit which broke it and resend?

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