linux-kernel - Re: [PATCH] Re: kswapd stuck using 100% CPU

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Date:	Sun, 25 Mar 2012 12:16:26 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Rik van Riel <riel@...hat.com>
cc:	Anton Blanchard <anton@...ba.org>, aarcange@...hat.com,
	mel@....ul.ie, akpm@...ux-foundation.org,
	lkml <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] Re: kswapd stuck using 100% CPU

On Sat, 24 Mar 2012, Rik van Riel wrote:
> 
> Only test compaction_suitable if the kernel is built with CONFIG_COMPACTION,
> otherwise the stub compaction_suitable function will always return
> COMPACT_SKIPPED and send kswapd into an infinite loop.
> 
> Signed-off-by: Rik van Riel <riel@...hat.com>
> Reported-by: Anton Blanchard <anton@...ba.org>

Thank you, Anton and Rik.  I never quite got around to investigating
why swapping had been nearly twice as slow with linux-next on my Aspire
One (with a relatively minimal config, omitting COMPACTION).  That was
the reason (one half of the HT cpu busy in kswapd), and this fixes it.

Tested-by: Hugh Dickins <hughd@...gle.com>

> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 7658fd6..33c332b 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2946,7 +2946,8 @@ out:
>  				continue;
>  
>  			/* Would compaction fail due to lack of free memory? */
> -			if (compaction_suitable(zone, order) == COMPACT_SKIPPED)
> +			if (COMPACTION_BUILD &&
> +			    compaction_suitable(zone, order) == COMPACT_SKIPPED)
>  				goto loop_again;
>  
>  			/* Confirm the zone is balanced for order-0 */
--
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