linux-kernel - [PATCH v2 2/2] block: kill extra rcu lock/unlock in queue enter

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Message-Id: <6b11c67ea495ed9d44f067622d852de4a510ce65.1634822969.git.asml.silence@gmail.com>
Date:   Thu, 21 Oct 2021 14:30:52 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     linux-block@...r.kernel.org
Cc:     Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Christoph Lameter <cl@...ux.com>,
        Tejun Heo <tj@...nel.org>, Dennis Zhou <dennis@...nel.org>,
        Pavel Begunkov <asml.silence@...il.com>
Subject: [PATCH v2 2/2] block: kill extra rcu lock/unlock in queue enter

blk_try_enter_queue() already takes rcu_read_lock/unlock, so we can
avoid the second pair in percpu_ref_tryget_live(), use a newly added
percpu_ref_tryget_live_rcu().

As rcu_read_lock/unlock imply barrier()s, it's pretty noticeable,
especially for for !CONFIG_PREEMPT_RCU (default for some distributions),
where __rcu_read_lock/unlock() are not inlined.

3.20%  io_uring  [kernel.vmlinux]  [k] __rcu_read_unlock
3.05%  io_uring  [kernel.vmlinux]  [k] __rcu_read_lock

2.52%  io_uring  [kernel.vmlinux]  [k] __rcu_read_unlock
2.28%  io_uring  [kernel.vmlinux]  [k] __rcu_read_lock

Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
---
 block/blk-core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 88752e51d2b6..20e76aeb50f5 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -389,7 +389,7 @@ EXPORT_SYMBOL(blk_cleanup_queue);
 static bool blk_try_enter_queue(struct request_queue *q, bool pm)
 {
 	rcu_read_lock();
-	if (!percpu_ref_tryget_live(&q->q_usage_counter))
+	if (!percpu_ref_tryget_live_rcu(&q->q_usage_counter))
 		goto fail;
 
 	/*
-- 
2.33.1