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

[Vlastimil Babka]

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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