linux-kernel - Re: [PATCH 04/12] mm, page_alloc: Only check cpusets when one exists that can be mem-controlled

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Date:	Tue, 25 Aug 2015 13:09:35 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Mel Gorman <mgorman@...hsingularity.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Michal Hocko <mhocko@...nel.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/12] mm, page_alloc: Only check cpusets when one exists
 that can be mem-controlled

On 08/25/2015 12:33 PM, Mel Gorman wrote:
> On Mon, Aug 24, 2015 at 10:53:37PM +0200, Vlastimil Babka wrote:
>> On 24.8.2015 15:16, Mel Gorman wrote:
>>>>>
>>>>>   	return read_seqcount_retry(&current->mems_allowed_seq, seq);
>>>>> @@ -139,7 +141,7 @@ static inline void set_mems_allowed(nodemask_t nodemask)
>>>>>
>>>>>   #else /* !CONFIG_CPUSETS */
>>>>>
>>>>> -static inline bool cpusets_enabled(void) { return false; }
>>>>> +static inline bool cpusets_mems_enabled(void) { return false; }
>>>>>
>>>>>   static inline int cpuset_init(void) { return 0; }
>>>>>   static inline void cpuset_init_smp(void) {}
>>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>>> index 62ae28d8ae8d..2c1c3bf54d15 100644
>>>>> --- a/mm/page_alloc.c
>>>>> +++ b/mm/page_alloc.c
>>>>> @@ -2470,7 +2470,7 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
>>>>>   		if (IS_ENABLED(CONFIG_NUMA) && zlc_active &&
>>>>>   			!zlc_zone_worth_trying(zonelist, z, allowednodes))
>>>>>   				continue;
>>>>> -		if (cpusets_enabled() &&
>>>>> +		if (cpusets_mems_enabled() &&
>>>>>   			(alloc_flags & ALLOC_CPUSET) &&
>>>>>   			!cpuset_zone_allowed(zone, gfp_mask))
>>>>>   				continue;
>>>>
>>>> Here the benefits are less clear. I guess cpuset_zone_allowed() is
>>>> potentially costly...
>>>>
>>>> Heck, shouldn't we just start the static key on -1 (if possible), so that
>>>> it's enabled only when there's 2+ cpusets?
>>
>> Hm wait a minute, that's what already happens:
>>
>> static inline int nr_cpusets(void)
>> {
>>          /* jump label reference count + the top-level cpuset */
>>          return static_key_count(&cpusets_enabled_key) + 1;
>> }
>>
>> I.e. if there's only the root cpuset, static key is disabled, so I think this
>> patch is moot after all?
>>
>
> static_key_count is an atomic read on a field in struct static_key where
> as static_key_false is a arch_static_branch which can be eliminated. The
> patch eliminates an atomic read so I didn't think it was moot.

Sorry I wasn't clear enough. My point is that AFAICS cpusets_enabled() 
will only return true if there are more cpusets than the root 
(top-level) one.
So the current cpusets_enabled() checks should be enough. Checking that 
"nr_cpusets() > 1" only duplicates what is already covered by 
cpusets_enabled() - see the nr_cpusets() listing above. I.e. David's 
premise was wrong.


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