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:	Wed, 26 Aug 2015 14:41:41 +0100
From:	Mel Gorman <mgorman@...hsingularity.net>
To:	Vlastimil Babka <vbabka@...e.cz>
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 Tue, Aug 25, 2015 at 01:09:35PM +0200, Vlastimil Babka wrote:
> 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.
> 

/me slaps self

I should have spotted that. Thanks.

-- 
Mel Gorman
SUSE Labs
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