linux-kernel - [PATCH 8/9] io-wq: optimise double lock for io_get_next

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Date:   Sun,  1 Mar 2020 19:18:25 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 8/9] io-wq: optimise double lock for io_get_next_work()

When executing non-linked hashed work, io_worker_handle_work()
will lock-unlock wqe->lock to update hash, and then immediately
lock-unlock to get next work. Optimise this case and lock/unlock
only once.

Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
---
 fs/io-wq.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/io-wq.c b/fs/io-wq.c
index da67c931db79..f9b18c16ebd8 100644
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -474,11 +474,11 @@ static void io_worker_handle_work(struct io_worker *worker)
 {
 	struct io_wqe *wqe = worker->wqe;
 	struct io_wq *wq = wqe->wq;
+	unsigned hash = -1U;
 
 	do {
 		struct io_wq_work *work;
-		unsigned hash = -1U;
-
+get_next:
 		/*
 		 * If we got some work, mark us as busy. If we didn't, but
 		 * the list isn't empty, it means we stalled on hashed work.
@@ -528,6 +528,9 @@ static void io_worker_handle_work(struct io_worker *worker)
 				wqe->flags &= ~IO_WQE_FLAG_STALLED;
 				/* dependent work is not hashed */
 				hash = -1U;
+				/* skip unnecessary unlock-lock wqe->lock */
+				if (!work)
+					goto get_next;
 				spin_unlock_irq(&wqe->lock);
 			}
 		} while (work);
-- 
2.24.0