[PATCH v4 2/2] io_uring: batch getting pcpu references

[Pavel Begunkov]

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

~5% throughput boost for a "submit and wait 128 nops" benchmark.

Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
---
 fs/io_uring.c | 26 +++++++++++++++++---------
 1 file changed, 17 insertions(+), 9 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 7fc1158bf9a4..404946080e86 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1080,9 +1080,6 @@ 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))
@@ -1141,6 +1138,14 @@ static void io_free_req_many(struct io_ring_ctx *ctx, void **reqs, int *nr)
 	}
 }
 
+static void __io_req_free_empty(struct io_kiocb *req)
+{
+	if (likely(!io_is_fallback_req(req)))
+		kmem_cache_free(req_cachep, req);
+	else
+		clear_bit_unlock(0, (unsigned long *) req->ctx->fallback_req);
+}
+
 static void __io_free_req(struct io_kiocb *req)
 {
 	struct io_ring_ctx *ctx = req->ctx;
@@ -1162,11 +1167,9 @@ static void __io_free_req(struct io_kiocb *req)
 			wake_up(&ctx->inflight_wait);
 		spin_unlock_irqrestore(&ctx->inflight_lock, flags);
 	}
-	percpu_ref_put(&ctx->refs);
-	if (likely(!io_is_fallback_req(req)))
-		kmem_cache_free(req_cachep, req);
-	else
-		clear_bit_unlock(0, (unsigned long *) ctx->fallback_req);
+
+	percpu_ref_put(&req->ctx->refs);
+	__io_req_free_empty(req);
 }
 
 static bool io_link_cancel_timeout(struct io_kiocb *req)
@@ -4551,6 +4554,9 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
 			return -EBUSY;
 	}
 
+	if (!percpu_ref_tryget_many(&ctx->refs, nr))
+		return -EAGAIN;
+
 	if (nr > IO_PLUG_THRESHOLD) {
 		io_submit_state_start(&state, nr);
 		statep = &state;
@@ -4567,7 +4573,7 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
 			break;
 		}
 		if (!io_get_sqring(ctx, req, &sqe)) {
-			__io_free_req(req);
+			__io_req_free_empty(req);
 			break;
 		}
 
@@ -4598,6 +4604,8 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
 			break;
 	}
 
+	if (submitted != nr)
+		percpu_ref_put_many(&ctx->refs, nr - submitted);
 	if (link)
 		io_queue_link_head(link);
 	if (statep)
-- 
2.24.0