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:	Mon, 24 Aug 2015 22:53:37 +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 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?

> It's overkill for the amount of benefit.
> 

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