linux-kernel - Re: [PATCH] mm, page_alloc: don't check cpuset allowed twice in fast-path

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Message-ID: <20170106104048.GB5561@dhcp22.suse.cz>
Date:   Fri, 6 Jan 2017 11:40:48 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm, page_alloc: don't check cpuset allowed twice in
 fast-path

On Fri 06-01-17 09:18:05, Vlastimil Babka wrote:
> Since commit 682a3385e773 ("mm, page_alloc: inline the fast path of the
> zonelist iterator") we replace a NULL nodemask with cpuset_current_mems_allowed
> in the fast path, so that get_page_from_freelist() filters nodes allowed by the
> cpuset via for_next_zone_zonelist_nodemask(). In that case it's pointless to
> also check __cpuset_zone_allowed(), which we can avoid by not using
> ALLOC_CPUSET in that scenario.

OK, this seems to be really worth it as most allocations go via
__alloc_pages so we can save __cpuset_zone_allowed in the fast path.

I was about to object how fragile this might be wrt. other ALLOC_CPUSET
checks but then I've realized this is only for the hotpath as the
slowpath goes through gfp_to_alloc_flags() which sets it back on.

Maybe all that could be added to the changelog?
 
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  mm/page_alloc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 2c6d5f64feca..3d86fbe2f4f4 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3754,9 +3754,10 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
>  
>  	if (cpusets_enabled()) {
>  		alloc_mask |= __GFP_HARDWALL;
> -		alloc_flags |= ALLOC_CPUSET;
>  		if (!ac.nodemask)
>  			ac.nodemask = &cpuset_current_mems_allowed;
> +		else
> +			alloc_flags |= ALLOC_CPUSET;
>  	}
>  
>  	gfp_mask &= gfp_allowed_mask;
> -- 
> 2.11.0
> 
> --
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-- 
Michal Hocko
SUSE Labs