linux-kernel - Re: [PATCH 3/9] mm, page_alloc: do not set_cpu_numa

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Date:   Fri, 14 Jul 2017 12:50:04 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Mel Gorman <mgorman@...e.de>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/9] mm, page_alloc: do not set_cpu_numa_mem on empty
 nodes initialization

On Fri 14-07-17 10:48:10, Mel Gorman wrote:
> On Fri, Jul 14, 2017 at 10:00:00AM +0200, Michal Hocko wrote:
> > From: Michal Hocko <mhocko@...e.com>
> > 
> > __build_all_zonelists reinitializes each online cpu local node for
> > CONFIG_HAVE_MEMORYLESS_NODES. This makes sense because previously memory
> > less nodes could gain some memory during memory hotplug and so the local
> > node should be changed for CPUs close to such a node. It makes less
> > sense to do that unconditionally for a newly creaded NUMA node which is
> > still offline and without any memory.
> > 
> > Let's also simplify the cpu loop and use for_each_online_cpu instead of
> > an explicit cpu_online check for all possible cpus.
> > 
> > Signed-off-by: Michal Hocko <mhocko@...e.com>
> > ---
> >  mm/page_alloc.c | 6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 7746824a425d..ebc3311555b1 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -5096,10 +5096,8 @@ static int __build_all_zonelists(void *data)
> >  
> >  			build_zonelists(pgdat);
> >  		}
> > -	}
> >  
> >  #ifdef CONFIG_HAVE_MEMORYLESS_NODES
> > -	for_each_possible_cpu(cpu) {
> >  		/*
> >  		 * We now know the "local memory node" for each node--
> >  		 * i.e., the node of the first zone in the generic zonelist.
> > @@ -5108,10 +5106,10 @@ static int __build_all_zonelists(void *data)
> >  		 * secondary cpus' numa_mem as they come on-line.  During
> >  		 * node/memory hotplug, we'll fixup all on-line cpus.
> >  		 */
> > -		if (cpu_online(cpu))
> > +		for_each_online_cpu(cpu)
> >  			set_cpu_numa_mem(cpu, local_memory_node(cpu_to_node(cpu)));
> > -	}
> >  #endif
> > +	}
> >  
> 
> This is not as clear a benefit. For each online node, we now go through
> all online CPUs once per node. There would be some rationale for using
> for_each_online_cpu.

I am not sure I understand. I am using for_each_online_cpu...
-- 
Michal Hocko
SUSE Labs