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 08:30:41 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Linux-MM <linux-mm@...ck.org>, Jiri Slaby <jslaby@...e.cz>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	Zlatko Calusic <zcalusic@...sync.net>,
	Johannes Weiner <hannes@...xchg.org>,
	dormando <dormando@...ia.net>,
	Satoru Moriya <satoru.moriya@....com>,
	Michal Hocko <mhocko@...e.cz>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 05/10] mm: vmscan: Do not allow kswapd to scan at maximum
 priority

On 03/21/2013 06:12 AM, Mel Gorman wrote:
> On Wed, Mar 20, 2013 at 09:20:14PM -0400, Rik van Riel wrote:
>> On 03/17/2013 09:04 AM, 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.
>>>
>>> Signed-off-by: Mel Gorman <mgorman@...e.de>
>>> ---
>>>   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:
>>>
>>
>> If priority 0 is way way way way way too aggressive, what makes
>> priority 1 safe?
>>
>
> The fact that priority 1 selects a sensible number of pages to reclaim and
> obeys swappiness makes it a lot safer. Priority 0 does this in get_scan_count
   ^^^^^^^^^^^^^^^^

Ahhh, good point!  We stay away from all the "emergency" code, which
kswapd should never run.

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

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