linux-kernel - [PATCH 2/2] mm, page_alloc: don't check cpuset allowed twice in fast-path

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Date:   Tue, 24 Jan 2017 16:05:11 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Michal Hocko <mhocko@...nel.org>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: [PATCH 2/2] mm, page_alloc: don't check cpuset allowed twice in fast-path

Since commit 682a3385e773 ("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.

Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
Acked-by: Michal Hocko <mhocko@...e.com>
Reviewed-by: Anshuman Khandual <khandual@...ux.vnet.ibm.com>
---
 mm/page_alloc.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 05068adf9007..407e5d89ad2e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3900,9 +3900,10 @@ 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);
-- 
2.11.0