linux-kernel - Re: [PATCH 08/31] mm, vmscan: simplify the logic deciding whether kswapd sleeps

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Date:	Wed, 6 Jul 2016 09:30:54 +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 08/31] mm, vmscan: simplify the logic deciding whether
 kswapd sleeps

On Tue, Jul 05, 2016 at 11:26:39AM +0100, Mel Gorman wrote:

<snip>

> > > @@ -3418,10 +3426,10 @@ void wakeup_kswapd(struct zone *zone, int order, enum zone_type classzone_idx)
> > >  	if (!cpuset_zone_allowed(zone, GFP_KERNEL | __GFP_HARDWALL))
> > >  		return;
> > >  	pgdat = zone->zone_pgdat;
> > > -	if (pgdat->kswapd_max_order < order) {
> > > -		pgdat->kswapd_max_order = order;
> > > -		pgdat->classzone_idx = min(pgdat->classzone_idx, classzone_idx);
> > > -	}
> > > +	if (pgdat->kswapd_classzone_idx == -1)
> > > +		pgdat->kswapd_classzone_idx = classzone_idx;
> > 
> > It's tricky. Couldn't we change kswapd_classzone_idx to integer type
> > and remove if above if condition?
> > 
> 
> It's tricky and not necessarily better overall. It's perfectly possible
> to be woken up for zone index 0 so it's changing -1 to another magic
> value.

I don't get it. What is a problem with this?

diff --git a/mm/vmscan.c b/mm/vmscan.c
index c538a8c..6eb23f5 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3413,9 +3413,7 @@ void wakeup_kswapd(struct zone *zone, int order, enum zone_type classzone_idx)
 	if (!cpuset_zone_allowed(zone, GFP_KERNEL | __GFP_HARDWALL))
 		return;
 	pgdat = zone->zone_pgdat;
-	if (pgdat->kswapd_classzone_idx == -1)
-		pgdat->kswapd_classzone_idx = classzone_idx;
-	pgdat->kswapd_classzone_idx = max(pgdat->kswapd_classzone_idx, classzone_idx);
+	pgdat->kswapd_classzone_idx = max_t(int, pgdat->kswapd_classzone_idx, classzone_idx);
 	pgdat->kswapd_order = max(pgdat->kswapd_order, order);
 	if (!waitqueue_active(&pgdat->kswapd_wait))
 		return;