Re: ppc: RECLAIM_DISTANCE 10?

[Nishanth Aravamudan <nacc@linux.vnet.ibm.com>]

Hi Michal,

On 18.02.2014 [10:06:58 +0100], Michal Hocko wrote:
> Hi,
> I have just noticed that ppc has RECLAIM_DISTANCE reduced to 10 set by
> 56608209d34b (powerpc/numa: Set a smaller value for RECLAIM_DISTANCE to
> enable zone reclaim). The commit message suggests that the zone reclaim
> is desirable for all NUMA configurations.
> 
> History has shown that the zone reclaim is more often harmful than
> helpful and leads to performance problems. The default RECLAIM_DISTANCE
> for generic case has been increased from 20 to 30 around 3.0
> (32e45ff43eaf mm: increase RECLAIM_DISTANCE to 30).

Interesting.

> I strongly suspect that the patch is incorrect and it should be
> reverted. Before I will send a revert I would like to understand what
> led to the patch in the first place. I do not see why would PPC use only
> LOCAL_DISTANCE and REMOTE_DISTANCE distances and in fact machines I have
> seen use different values.
> 
> Anton, could you comment please?

I'll let Anton comment here, but in looking into this issue in working
on CONFIG_HAVE_MEMORYLESS_NODE support, I realized that any LPAR with
memoryless nodes will set zone_reclaim_mode to 1. I think we want to
ignore memoryless nodes when we set up the reclaim mode like the
following? I'll send it as a proper patch if you agree?

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 5de4337..4f6ff6f 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1853,8 +1853,9 @@ static void __paginginit init_zone_allows_reclaim(int nid)
 {
        int i;
 
-       for_each_online_node(i)
-               if (node_distance(nid, i) <= RECLAIM_DISTANCE)
+       for_each_online_node(i) {
+               if (node_distance(nid, i) <= RECLAIM_DISTANCE ||
+                                       local_memory_node(nid) != nid)
                        node_set(i, NODE_DATA(nid)->reclaim_nodes);
                else
                        zone_reclaim_mode = 1;

Note, this won't actually do anything if CONFIG_HAVE_MEMORYLESS_NODES is
not set, but if it is, I think semantically it will indicate that
memoryless nodes *have* to reclaim remotely.

And actually the above won't work, because the callpath is

start_kernel -> setup_arch -> paging_init [-> free_area_init_nodes ->
free_area_init_node -> init_zone_allows_reclaim] which is called before
build_all_zonelists. This is a similar ordering problem as I'm having
with the MEMORYLESS_NODE support, will work on it.

Thanks,
Nish