linux-kernel - [PATCH 2/2] io_uring: batch getting pcpu references

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Date:   Wed, 18 Dec 2019 01:28:39 +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 2/2] io_uring: batch getting pcpu references

percpu_ref_tryget() has its own overhead. Instead getting a reference
for each request, grab a bunch once per io_submit_sqes().

basic benchmark with submit and wait 128 non-linked nops showed ~5%
performance gain. (7044 KIOPS vs 7423)

Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
---

For notice: it could be done without @extra_refs variable,
but looked too tangled because of gotos.


 fs/io_uring.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index cf4138f0e504..6c85dfc62224 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -845,9 +845,6 @@ static struct io_kiocb *io_get_req(struct io_ring_ctx *ctx,
 	gfp_t gfp = GFP_KERNEL | __GFP_NOWARN;
 	struct io_kiocb *req;
 
-	if (!percpu_ref_tryget(&ctx->refs))
-		return NULL;
-
 	if (!state) {
 		req = kmem_cache_alloc(req_cachep, gfp);
 		if (unlikely(!req))
@@ -3929,6 +3926,7 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
 	struct io_submit_state state, *statep = NULL;
 	struct io_kiocb *link = NULL;
 	int i, submitted = 0;
+	unsigned int extra_refs;
 	bool mm_fault = false;
 
 	/* if we have a backlog and couldn't flush it all, return BUSY */
@@ -3941,6 +3939,10 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
 		statep = &state;
 	}
 
+	if (!percpu_ref_tryget_many(&ctx->refs, nr))
+		return -EAGAIN;
+	extra_refs = nr;
+
 	for (i = 0; i < nr; i++) {
 		struct io_kiocb *req = io_get_req(ctx, statep);
 
@@ -3949,6 +3951,7 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
 				submitted = -EAGAIN;
 			break;
 		}
+		--extra_refs;
 		if (!io_get_sqring(ctx, req)) {
 			__io_free_req(req);
 			break;
@@ -3976,6 +3979,8 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
 		io_queue_link_head(link);
 	if (statep)
 		io_submit_state_end(&state);
+	if (extra_refs)
+		percpu_ref_put_many(&ctx->refs, extra_refs);
 
 	 /* Commit SQ ring head once we've consumed and submitted all SQEs */
 	io_commit_sqring(ctx);
-- 
2.24.0