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

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Message-ID: <20170714094810.ftthctfz33artwh2@suse.de>
Date:   Fri, 14 Jul 2017 10:48:10 +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>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 3/9] mm, page_alloc: do not set_cpu_numa_mem on empty
 nodes initialization

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.

-- 
Mel Gorman
SUSE Labs