linux-kernel - Re: [PATCH 01/31] mm, vmstat: add infrastructure for per-node vmstats

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Date:	Wed, 6 Jul 2016 09:15:40 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Mel Gorman <mgorman@...hsingularity.net>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>, Rik van Riel <riel@...riel.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/31] mm, vmstat: add infrastructure for per-node vmstats

On Tue, Jul 05, 2016 at 09:14:05AM +0100, Mel Gorman wrote:
> On Tue, Jul 05, 2016 at 08:50:18AM +0900, Minchan Kim wrote:
> > > @@ -172,13 +174,17 @@ void refresh_zone_stat_thresholds(void)
> > >  	int threshold;
> > >  
> > >  	for_each_populated_zone(zone) {
> > > +		struct pglist_data *pgdat = zone->zone_pgdat;
> > >  		unsigned long max_drift, tolerate_drift;
> > >  
> > >  		threshold = calculate_normal_threshold(zone);
> > >  
> > > -		for_each_online_cpu(cpu)
> > > +		for_each_online_cpu(cpu) {
> > >  			per_cpu_ptr(zone->pageset, cpu)->stat_threshold
> > >  							= threshold;
> > > +			per_cpu_ptr(pgdat->per_cpu_nodestats, cpu)->stat_threshold
> > > +							= threshold;
> > > +		}
> > 
> > I didn't see other patches yet so it might fix it then.
> > 
> > per_cpu_nodestats is per node not zone but it use per-zone threshold
> > and even overwritten by next zones. I don't think it's not intended.
> 
> It was intended that the threshold from one zone would be used but now
> that you point it out, it would use the threshold for the smallest zone
> in the node which is sub-optimal. I applied the patch below on top to
> use the threshold from the largest zone. I considered using the sum of
> all thresholds but feared it might allow too much per-cpu drift. It can
> be switched to the sum if we find a case where vmstat updates are too
> high.

Fair enough.