linux-kernel - [PATCH 3/5] percpu_ida: Optimize freeing tags when maximum cache size is 1

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Date:	Mon, 28 Oct 2013 11:05:48 +0100
From:	Alexander Gordeev <agordeev@...hat.com>
To:	Kent Overstreet <kmo@...erainc.com>
Cc:	Oleg Nesterov <oleg@...hat.com>, Jens Axboe <axboe@...nel.dk>,
	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 3/5] percpu_ida: Optimize freeing tags when maximum cache
 size is 1

In case percpu_max_size is 1 percpu_ida_free() sets bit in
the cpumask and immediately transfers the tag to the pool.
As result, the very next call to percpu_ida_alloc() on the
same CPU will have to pull a tag from the pool to the local
cache and so on. Hence, positive effects of local caching
become largely negated.

This update assumes stealing tags is faster than ping-ponging
between local caches and the pool and prevents returning tags
to the pool in case percpu_max_size is 1.

Signed-off-by: Alexander Gordeev <agordeev@...hat.com>
---
 lib/percpu_ida.c |    5 ++---
 1 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/lib/percpu_ida.c b/lib/percpu_ida.c
index 9dd8741..4adc3e5 100644
--- a/lib/percpu_ida.c
+++ b/lib/percpu_ida.c
@@ -243,9 +243,8 @@ void percpu_ida_free(struct percpu_ida *pool, unsigned tag)
 		 */
 		smp_wmb();
 		wake_up = true;
-	} 
-
-	if (nr_free == pool->percpu_max_size) {
+	} else if ((nr_free == pool->percpu_max_size) &&
+		   (pool->percpu_max_size > 1)) {
 		spin_lock(&pool->lock);
 
 		/*
-- 
1.7.7.6


-- 
Regards,
Alexander Gordeev
agordeev@...hat.com
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