linux-kernel - Re: [PATCH 20/31] mm, vmscan: only wakeup kswapd once per node for the requested classzone

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Date:	Thu, 7 Jul 2016 11:58:09 +0100
From:	Mel Gorman <mgorman@...hsingularity.net>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
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 20/31] mm, vmscan: only wakeup kswapd once per node for
 the requested classzone

On Thu, Jul 07, 2016 at 10:24:23AM +0900, Joonsoo Kim wrote:
> On Fri, Jul 01, 2016 at 09:01:28PM +0100, Mel Gorman wrote:
> > kswapd is woken when zones are below the low watermark but the wakeup
> > decision is not taking the classzone into account.  Now that reclaim is
> > node-based, it is only required to wake kswapd once per node and only if
> > all zones are unbalanced for the requested classzone.
> > 
> > Note that one node might be checked multiple times if the zonelist is
> > ordered by node because there is no cheap way of tracking what nodes have
> > already been visited.  For zone-ordering, each node should be checked only
> > once.
> > 
> > Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> > Acked-by: Vlastimil Babka <vbabka@...e.cz>
> > ---
> >  mm/page_alloc.c |  8 ++++++--
> >  mm/vmscan.c     | 13 +++++++++++--
> >  2 files changed, 17 insertions(+), 4 deletions(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 441f482bf9a2..2fe2fbb4f2ad 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -3410,10 +3410,14 @@ static void wake_all_kswapds(unsigned int order, const struct alloc_context *ac)
> >  {
> >  	struct zoneref *z;
> >  	struct zone *zone;
> > +	pg_data_t *last_pgdat = NULL;
> >  
> >  	for_each_zone_zonelist_nodemask(zone, z, ac->zonelist,
> > -						ac->high_zoneidx, ac->nodemask)
> > -		wakeup_kswapd(zone, order, ac_classzone_idx(ac));
> > +					ac->high_zoneidx, ac->nodemask) {
> > +		if (last_pgdat != zone->zone_pgdat)
> > +			wakeup_kswapd(zone, order, ac_classzone_idx(ac));
> > +		last_pgdat = zone->zone_pgdat;
> > +	}
> >  }
> 
> In wakeup_kswapd(), there is a check if it is a populated zone or not.

It's redundant.

> If first zone in node is not a populated zone, wakeup_kswapd() would be
> skipped. Though, I'm not sure if zonelist can include a un-populated
> zone.

Zonelists do not contain unpopulated zones.

> Perhaps, moving populated zone check in wakeup_kswapd() to here
> would be a safe code.
> 

If anything was going to happen to it, it should be deleted. It's a
minor cleanup.

-- 
Mel Gorman
SUSE Labs