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* [PATCH] io_uring: add likely/unlikely in io_get_sqring()
@ 2019-11-21 18:24 Pavel Begunkov
  0 siblings, 0 replies; only message in thread
From: Pavel Begunkov @ 2019-11-21 18:24 UTC (permalink / raw)
  To: Jens Axboe, io-uring

The number of SQEs to submit is specified by a user, so io_get_sqring()
in most of the cases succeeds. Hint compilers about that.

Checking ASM genereted by gcc 9.2.0 for x64, there is one branch
misprediction.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 fs/io_uring.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index fa1cf7263959..86d1d8f272ae 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -3112,11 +3112,11 @@ static bool io_get_sqring(struct io_ring_ctx *ctx, struct sqe_submit *s)
 	 */
 	head = ctx->cached_sq_head;
 	/* make sure SQ entry isn't read before tail */
-	if (head == smp_load_acquire(&rings->sq.tail))
+	if (unlikely(head == smp_load_acquire(&rings->sq.tail)))
 		return false;
 
 	head = READ_ONCE(sq_array[head & ctx->sq_mask]);
-	if (head < ctx->sq_entries) {
+	if (likely(head < ctx->sq_entries)) {
 		s->ring_file = NULL;
 		s->sqe = &ctx->sq_sqes[head];
 		s->sequence = ctx->cached_sq_head;
-- 
2.24.0


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2019-11-21 18:24 [PATCH] io_uring: add likely/unlikely in io_get_sqring() Pavel Begunkov

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