linux-kernel - Re: [PATCH 1/4] mm: vmscan: Correct check for kswapd sleeping in sleeping

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Date:	Sat, 25 Jun 2011 17:33:39 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Pádraig Brady <P@...igBrady.com>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Colin King <colin.king@...onical.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Andrew Lutomirski <luto@....edu>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] mm: vmscan: Correct check for kswapd sleeping in
 sleeping_prematurely

On 06/24/2011 10:44 AM, Mel Gorman wrote:
> During allocator-intensive workloads, kswapd will be woken frequently
> causing free memory to oscillate between the high and min watermark.
> This is expected behaviour.
>
> A problem occurs if the highest zone is small.  balance_pgdat()
> only considers unreclaimable zones when priority is DEF_PRIORITY
> but sleeping_prematurely considers all zones. It's possible for this
> sequence to occur
>
>    1. kswapd wakes up and enters balance_pgdat()
>    2. At DEF_PRIORITY, marks highest zone unreclaimable
>    3. At DEF_PRIORITY-1, ignores highest zone setting end_zone
>    4. At DEF_PRIORITY-1, calls shrink_slab freeing memory from
>          highest zone, clearing all_unreclaimable. Highest zone
>          is still unbalanced
>    5. kswapd returns and calls sleeping_prematurely
>    6. sleeping_prematurely looks at *all* zones, not just the ones
>       being considered by balance_pgdat. The highest small zone
>       has all_unreclaimable cleared but but the zone is not
>       balanced. all_zones_ok is false so kswapd stays awake
>
> This patch corrects the behaviour of sleeping_prematurely to check
> the zones balance_pgdat() checked.
>
> Reported-and-tested-by: Pádraig Brady<P@...igBrady.com>
> Signed-off-by: Mel Gorman<mgorman@...e.de>

Acked-by: Rik van Riel <riel@...hat.com>

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