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From: Vlastimil Babka <vbabka@suse.cz>
To: Mel Gorman <mgorman@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Linux Kernel <linux-kernel@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>,
	Linux-FSDevel <linux-fsdevel@vger.kernel.org>,
	Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH 6/6] mm: page_alloc: Reduce cost of the fair zone allocation policy
Date: Mon, 11 Aug 2014 14:34:05 +0200	[thread overview]
Message-ID: <53E8B83D.1070004@suse.cz> (raw)
In-Reply-To: <20140811121241.GD7970@suse.de>

On 08/11/2014 02:12 PM, Mel Gorman wrote:
> On Fri, Aug 08, 2014 at 05:27:15PM +0200, Vlastimil Babka wrote:
>> On 07/09/2014 10:13 AM, Mel Gorman wrote:
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -1604,6 +1604,9 @@ again:
>>>   	}
>>>
>>>   	__mod_zone_page_state(zone, NR_ALLOC_BATCH, -(1 << order));
>>
>> This can underflow zero, right?
>>
>
> Yes, because of per-cpu accounting drift.

I meant mainly because of order > 0.

>>> +	if (zone_page_state(zone, NR_ALLOC_BATCH) == 0 &&
>>
>> AFAICS, zone_page_state will correct negative values to zero only for
>> CONFIG_SMP. Won't this check be broken on !CONFIG_SMP?
>>
>
> On !CONFIG_SMP how can there be per-cpu accounting drift that would make
> that counter negative?

Well original code used "if (zone_page_state(zone, NR_ALLOC_BATCH) <= 
0)" elsewhere, that you are replacing with zone_is_fair_depleted check. 
I assumed it's because it can get negative due to order > 0. I might 
have not looked thoroughly enough but it seems to me there's nothing 
that would prevent it, such as skipping a zone because its remaining 
batch is lower than 1 << order.
So I think the check should be "<= 0" to be safe.

Vlastimil



  reply	other threads:[~2014-08-11 12:34 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-07-09  8:13 [PATCH 0/5] Reduce sequential read overhead Mel Gorman
2014-07-09  8:13 ` [PATCH 1/6] mm: pagemap: Avoid unnecessary overhead when tracepoints are deactivated Mel Gorman
2014-07-10 12:01   ` Johannes Weiner
2014-07-09  8:13 ` [PATCH 2/6] mm: Rearrange zone fields into read-only, page alloc, statistics and page reclaim lines Mel Gorman
2014-07-10 12:06   ` Johannes Weiner
2014-07-09  8:13 ` [PATCH 3/6] mm: Move zone->pages_scanned into a vmstat counter Mel Gorman
2014-07-10 12:08   ` Johannes Weiner
2014-07-09  8:13 ` [PATCH 4/6] mm: vmscan: Only update per-cpu thresholds for online CPU Mel Gorman
2014-07-10 12:09   ` Johannes Weiner
2014-07-09  8:13 ` [PATCH 5/6] mm: page_alloc: Abort fair zone allocation policy when remotes nodes are encountered Mel Gorman
2014-07-10 12:14   ` Johannes Weiner
2014-07-10 12:44     ` Mel Gorman
2014-07-09  8:13 ` [PATCH 6/6] mm: page_alloc: Reduce cost of the fair zone allocation policy Mel Gorman
2014-07-10 12:18   ` Johannes Weiner
2014-08-08 15:27   ` Vlastimil Babka
2014-08-11 12:12     ` Mel Gorman
2014-08-11 12:34       ` Vlastimil Babka [this message]
2014-09-02 14:01         ` Johannes Weiner
2014-09-05 10:14           ` [PATCH] mm: page_alloc: Fix setting of ZONE_FAIR_DEPLETED on UP Mel Gorman
2014-09-07  6:32             ` Leon Romanovsky
2014-09-08 11:57               ` [PATCH] mm: page_alloc: Fix setting of ZONE_FAIR_DEPLETED on UP v2 Mel Gorman
2014-09-09 19:53                 ` Andrew Morton
2014-09-10  9:16                   ` Mel Gorman
2014-09-10 20:32                     ` Johannes Weiner

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