mm, page_alloc: don't check cpuset allowed twice in fast-path

Since commit 682a3385e7 ("mm, page_alloc: inline the fast path of the
zonelist iterator") we replace a NULL nodemask with
cpuset_current_mems_allowed in the fast path, so that
get_page_from_freelist() filters nodes allowed by the cpuset via
for_next_zone_zonelist_nodemask().

In that case it's pointless to additionaly check __cpuset_zone_allowed()
in each iteration, which we can avoid by not adding ALLOC_CPUSET to
alloc_flags in that scenario.

This saves some cycles in the allocator fast path on systems with one or
more non-root cpuset configured.  In the slow path, ALLOC_CPUSET is
reset according to __alloc_pages_slowpath().  Without configured
cpusets, this code is disabled by a static key.

Link: http://lkml.kernel.org/r/20170124150511.5710-2-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -3903,9 +3903,10 @@ static inline bool prepare_alloc_pages(gfp_t gfp_mask, unsigned int order,
 
 	if (cpusets_enabled()) {
 		*alloc_mask |= __GFP_HARDWALL;
-		*alloc_flags |= ALLOC_CPUSET;
 		if (!ac->nodemask)
 			ac->nodemask = &cpuset_current_mems_allowed;
+		else
+			*alloc_flags |= ALLOC_CPUSET;
 	}
 
 	lockdep_trace_alloc(gfp_mask);