Date: Tue, 18 Feb 2014 16:40:54 -0800
From: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
To: Michal Hocko <mhocko@suse.cz>
Cc: linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org,
        Anton Blanchard <anton@samba.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: ppc: RECLAIM_DISTANCE 10?
Message-ID: <20140219004053.GA27108@linux.vnet.ibm.com>
References: <20140218090658.GA28130@dhcp22.suse.cz>
 <20140218233404.GB10844@linux.vnet.ibm.com>
 <20140218235800.GC10844@linux.vnet.ibm.com>
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On 18.02.2014 [15:58:00 -0800], Nishanth Aravamudan wrote:
> On 18.02.2014 [15:34:05 -0800], 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?
> > 
> > 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.
> 
> How about the following?
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 5de4337..1a0eced 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1854,7 +1854,8 @@ static void __paginginit init_zone_allows_reclaim(int nid)
>         int i;
>  
>         for_each_online_node(i)
> -               if (node_distance(nid, i) <= RECLAIM_DISTANCE)
> +               if (node_distance(nid, i) <= RECLAIM_DISTANCE ||
> +                                       !NODE_DATA(nid)->node_present_pages)

err s/nid/i/ above.

-Nish

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