linux-kernel - Re: [PATCH 05/10] mm: vmscan: Do not allow kswapd to scan at maximum priority

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Date:	Thu, 21 Mar 2013 16:48:10 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Linux-MM <linux-mm@...ck.org>, Jiri Slaby <jslaby@...e.cz>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	Rik van Riel <riel@...hat.com>,
	Zlatko Calusic <zcalusic@...sync.net>,
	Johannes Weiner <hannes@...xchg.org>,
	dormando <dormando@...ia.net>,
	Satoru Moriya <satoru.moriya@....com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 05/10] mm: vmscan: Do not allow kswapd to scan at maximum
 priority

On Sun 17-03-13 13:04:11, Mel Gorman wrote:
> Page reclaim at priority 0 will scan the entire LRU as priority 0 is
> considered to be a near OOM condition. Kswapd can reach priority 0 quite
> easily if it is encountering a large number of pages it cannot reclaim
> such as pages under writeback. When this happens, kswapd reclaims very
> aggressively even though there may be no real risk of allocation failure
> or OOM.
> 
> This patch prevents kswapd reaching priority 0 and trying to reclaim
> the world. Direct reclaimers will still reach priority 0 in the event
> of an OOM situation.

OK, it should work. raise_priority should prevent from pointless
lowerinng the priority and if there is really nothing to reclaim then
relying on the direct reclaim is probably a better idea.

> Signed-off-by: Mel Gorman <mgorman@...e.de>

Reviewed-by: Michal Hocko <mhocko@...e.cz>

> ---
>  mm/vmscan.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 7513bd1..af3bb6f 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2891,7 +2891,7 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>  		 */
>  		if (raise_priority || !this_reclaimed)
>  			sc.priority--;
> -	} while (sc.priority >= 0 &&
> +	} while (sc.priority >= 1 &&
>  		 !pgdat_balanced(pgdat, order, *classzone_idx));
>  
>  out:
> -- 
> 1.8.1.4
> 

-- 
Michal Hocko
SUSE Labs
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