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

[Minchan Kim]

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;