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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