linux-kernel - Re: kswapd0: excessive CPU usage

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Message-ID: <508E5FD3.1060105@leemhuis.info>
Date:	Mon, 29 Oct 2012 11:52:03 +0100
From:	Thorsten Leemhuis <fedora@...mhuis.info>
To:	Mel Gorman <mgorman@...e.de>
CC:	Jiri Slaby <jslaby@...e.cz>, Valdis.Kletnieks@...edu,
	Jiri Slaby <jirislaby@...il.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: kswapd0: excessive CPU usage

Hi!

On 15.10.2012 13:09, Mel Gorman wrote:
> On Mon, Oct 15, 2012 at 11:54:13AM +0200, Jiri Slaby wrote:
>> On 10/12/2012 03:57 PM, Mel Gorman wrote:
>>> mm: vmscan: scale number of pages reclaimed by reclaim/compaction only in direct reclaim
>>> Jiri Slaby reported the following:
 > [...]
>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>> index 2624edc..2b7edfa 100644
>>> --- a/mm/vmscan.c
>>> +++ b/mm/vmscan.c
>>> @@ -1763,14 +1763,20 @@ static bool in_reclaim_compaction(struct scan_control *sc)
>>>   #ifdef CONFIG_COMPACTION
>>>   /*
>>>    * If compaction is deferred for sc->order then scale the number of pages
>>> - * reclaimed based on the number of consecutive allocation failures
>>> + * reclaimed based on the number of consecutive allocation failures. This
>>> + * scaling only happens for direct reclaim as it is about to attempt
>>> + * compaction. If compaction fails, future allocations will be deferred
>>> + * and reclaim avoided. On the other hand, kswapd does not take compaction
>>> + * deferral into account so if it scaled, it could scan excessively even
>>> + * though allocations are temporarily not being attempted.
>>>    */
>>>   static unsigned long scale_for_compaction(unsigned long pages_for_compaction,
>>>   			struct lruvec *lruvec, struct scan_control *sc)
>>>   {
>>>   	struct zone *zone = lruvec_zone(lruvec);
>>>
>>> -	if (zone->compact_order_failed <= sc->order)
>>> +	if (zone->compact_order_failed <= sc->order &&
>>> +	    !current_is_kswapd())
>>>   		pages_for_compaction <<= zone->compact_defer_shift;
>>>   	return pages_for_compaction;
>>>   }
>> Yes, applying this instead of the revert fixes the issue as well.

Just wondering, is there a reason why this patch wasn't applied to 
mainline? Did it simply fall through the cracks? Or am I missing something?

I'm asking because I think I stil see the issue on 
3.7-rc2-git-checkout-from-friday. Seems Fedora rawhide users are hitting 
it, too:
https://bugzilla.redhat.com/show_bug.cgi?id=866988

Or are we seeing something different which just looks similar? I can 
test the patch if it needs further testing, but from the discussion I 
got the impression that everything is clear and the patch ready for merging.

CU
  knurd
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