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

[Mel Gorman <mgorman@techsingularity.net>]

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.

-- 
Mel Gorman
SUSE Labs