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Date:	Thu, 20 Mar 2014 21:33:58 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] mm: page_alloc: spill to remote nodes before waking
 kswapd

On Thu, Mar 20, 2014 at 04:53:36PM -0400, Johannes Weiner wrote:
> On NUMA systems, a node may start thrashing cache or even swap
> anonymous pages while there are still free pages on remote nodes.
> 
> This is a result of 81c0a2bb515f ("mm: page_alloc: fair zone allocator
> policy") and fff4068cba48 ("mm: page_alloc: revert NUMA aspect of fair
> allocation policy").  Before those changes, the allocator would first
> try all allowed zones, including those on remote nodes, before waking
> any kswapds.  But now, the allocator fastpath doubles as the fairness
> pass, which in turn can only consider the local node to prevent remote
> spilling based on exhausted fairness batches alone.  Remote nodes are
> only considered in the slowpath, after the kswapds are woken up.  But
> if remote nodes still have free memory, kswapd should not be woken to
> rebalance the local node or it may thrash cash or swap prematurely.
> 
> Fix this by adding one more unfair pass over the zonelist that is
> allowed to spill to remote nodes after the local fairness pass fails
> but before entering the slowpath and waking the kswapds.
> 
> This also gets rid of the GFP_THISNODE exemption from the fairness
> protocol because the unfair pass is no longer tied to kswapd, which
> GFP_THISNODE is not allowed to wake up.
> 
> However, because remote spills can be more frequent now - we prefer
> them over local kswapd reclaim - the allocation batches on remote
> nodes could underflow more heavily.  When resetting the batches, use
> atomic_long_read() directly instead of zone_page_state() to calculate
> the delta as the latter filters negative counter values.
> 
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> Cc: <stable@...nel.org> [3.12+]

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

-- 
Mel Gorman
SUSE Labs
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