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

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Date:   Fri, 14 Jul 2017 13:32:42 +0100
From:   Mel Gorman <mgorman@...e.de>
To:     Michal Hocko <mhocko@...nel.org>
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, Jul 14, 2017 at 12:50:04PM +0200, Michal Hocko wrote:
> 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...

Yes, but within a loop that looks like

for_each_online_node(nid)
	...
	for_each_online_cpu(cpu)

Or maybe you aren't because we are looking at different baselines. I had
minor fuzz and conflicts applying the series.

-- 
Mel Gorman
SUSE Labs