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

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Date:   Fri, 6 Jan 2017 11:47:36 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>
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 01/06/2017 11:40 AM, Michal Hocko wrote:
> 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.

Well the "really fast path" assumes that there are no cpusets (except
the root one), which is done using static key check in
cpusets_enabled(). But we can still do better even if they are enabled.

> 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?

OK, will do after collecting more feedback.

>  
>> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> 
> Acked-by: Michal Hocko <mhocko@...e.com>

Thanks!

> 
>> ---
>>  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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