linux-kernel - Re: [PATCH v2] mm: vmscan: do not throttle based on pfmemalloc reserves if node has no reclaimable pages

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Date:	Fri, 3 Apr 2015 20:24:45 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Nishanth Aravamudan <nacc@...ux.vnet.ibm.com>
Cc:	Dave Hansen <dave.hansen@...el.com>, Mel Gorman <mgorman@...e.de>,
	anton@...bar.org, linuxppc-dev@...ts.ozlabs.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>,
	Dan Streetman <ddstreet@...e.org>
Subject: Re: [PATCH v2] mm: vmscan: do not throttle based on pfmemalloc
 reserves if node has no reclaimable pages

On Fri 03-04-15 10:43:57, Nishanth Aravamudan wrote:
> On 31.03.2015 [11:48:29 +0200], Michal Hocko wrote:
[...]
> > I would expect kswapd would be looping endlessly because the zone
> > wouldn't be balanced obviously. But I would be wrong... because
> > pgdat_balanced is doing this:
> > 		/*
> > 		 * A special case here:
> > 		 *
> > 		 * balance_pgdat() skips over all_unreclaimable after
> > 		 * DEF_PRIORITY. Effectively, it considers them balanced so
> > 		 * they must be considered balanced here as well!
> > 		 */
> > 		if (!zone_reclaimable(zone)) {
> > 			balanced_pages += zone->managed_pages;
> > 			continue;
> > 		}
> > 
> > and zone_reclaimable is false for you as you didn't have any
> > zone_reclaimable_pages(). But wakeup_kswapd doesn't do this check so it
> > would see !zone_balanced() AFAICS (build_zonelists doesn't ignore those
> > zones right?) and so the kswapd would be woken up easily. So it looks
> > like a mess.
> 
> My understanding, and I could easily be wrong, is that kswapd2 (node 2
> is the exhausted one) spins endlessly, because the reclaim logic sees
> that we are reclaiming from somewhere but the allocation request for
> node 2 (which is __GFP_THISNODE for hugepages, not GFP_THISNODE) will
> never complete, so we just continue to reclaim.

__GFP_THISNODE would be waking up kswapd2 again and again, that is true.
I am just wondering whether we will have any __GFP_THISNODE allocations
for a node without CPUs (numa_node_id() shouldn't return such a node
AFAICS). Maybe if somebody is bound to Node2 explicitly but I would
consider this as a misconfiguration.

-- 
Michal Hocko
SUSE Labs
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