Re: [PATCH 3/7] mm: page_alloc: Use zone node IDs to approximatelocality

[Johannes Weiner]

On Tue, Dec 17, 2013 at 11:13:52AM +0000, Mel Gorman wrote:
> On Mon, Dec 16, 2013 at 03:25:07PM -0500, Johannes Weiner wrote:
> > On Fri, Dec 13, 2013 at 02:10:03PM +0000, Mel Gorman wrote:
> > > zone_local is using node_distance which is a more expensive call than
> > > necessary. On x86, it's another function call in the allocator fast path
> > > and increases cache footprint. This patch makes the assumption zones on a
> > > local node will share the same node ID. The necessary information should
> > > already be cache hot.
> > > 
> > > Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
> > > ---
> > >  mm/page_alloc.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > index 64020eb..fd9677e 100644
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -1816,7 +1816,7 @@ static void zlc_clear_zones_full(struct zonelist *zonelist)
> > >  
> > >  static bool zone_local(struct zone *local_zone, struct zone *zone)
> > >  {
> > > -	return node_distance(local_zone->node, zone->node) == LOCAL_DISTANCE;
> > > +	return zone_to_nid(zone) == numa_node_id();
> > 
> > Why numa_node_id()?  We pass in the preferred zone as @local_zone:
> > 
> 
> Initially because I was thinking "local node" and numa_node_id() is a
> per-cpu variable that should be cheap to access and in some cases
> cache-hot as the top-level gfp API calls numa_node_id().
> 
> Thinking about it more though it still makes sense because the preferred
> zone is not necessarily local. If the allocation request requires ZONE_DMA32
> and the local node does not have that zone then preferred zone is on a
> remote node.

Don't we treat everything in relation to the preferred zone?
zone_reclaim_mode itself does not compare with numa_node_id() but with
whatever is the preferred zone.

I could see some value in changing that to numa_node_id(), but then
zone_local() and zone_allows_reclaim() should probably both switch.
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