ppc: RECLAIM_DISTANCE 10?

[Michal Hocko]

On Tue 18-02-14 15:34:05, Nishanth Aravamudan wrote:
> 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?

Funny enough, ppc memoryless node setup is what led me to this code.
We had a setup like this:
node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
node 0 size: 0 MB
node 0 free: 0 MB
node 2 cpus:
node 2 size: 7168 MB
node 2 free: 6019 MB
node distances:
node   0   2
0:  10  40
2:  40  10

Which ends up enabling zone_reclaim although there is only a single node
with memory. Not that RECLAIM_DISTANCE would make any difference here as
the distance is even above default RECLAIM_DISTANCE.

> 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.

I think you just want for_each_node_state(nid, N_MEMORY) and skip all
the memory less nodes, no?
-- 
Michal Hocko
SUSE Labs