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

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Date:	Thu, 7 Jul 2016 14:51:21 +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 Wed, Jul 06, 2016 at 09:31:21AM +0100, Mel Gorman wrote:
> On Wed, Jul 06, 2016 at 09:30:54AM +0900, Minchan Kim wrote:
> > 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?
> > 
> 
> It becomes difficult to tell the difference between "no wakeup and init to
> zone 0" and "wakeup and reclaim for zone 0". At least that's the problem
> I ran into when I tried before settling on -1.

Sorry for bothering you several times. I cannot parse what you mean.
I didn't mean -1 is problem here but why do we need below two lines
I removed?

IOW, what's the problem if we apply below patch?

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;  

> 
> -- 
> Mel Gorman
> SUSE Labs