io_uring: optimise non-drain path

Replace drain checks with one-way flag set upon seeing the first
IOSQE_IO_DRAIN request. There are several places where it cuts cycles
well:

1) It's much faster than the fast check with two
conditions in io_drain_req() including pretty complex
list_empty_careful().

2) We can mark io_queue_sqe() inline now, that's a huge win.

3) It replaces timeout and drain checks in io_commit_cqring() with a
single flags test. Also great not touching ->defer_list there without a
reason so limiting cache bouncing.

It adds a small amount of overhead to drain path, but it's negligible.
The main nuisance is that once it meets any DRAIN request in io_uring
instance lifetime it will _always_ go through a slower path, so
drain-less and offset-mode timeout less applications are preferable.
The overhead in that case would be not big, but it's worth to bear in
mind.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/98d2fff8c4da5144bb0d08499f591d4768128ea3.1623709150.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index fc764e912844c81fde9c0c35165a5e2fd31c2e25..ab9a16c8954536cfd2d267bdedd2944372722433 100644 (file)
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -352,6 +352,7 @@ struct io_ring_ctx {
                unsigned int            eventfd_async: 1;
                unsigned int            restricted: 1;
                unsigned int            off_timeout_used: 1;
+               unsigned int            drain_used: 1;
        } ____cacheline_aligned_in_smp;
 
        /* submission data */
@@ -1299,9 +1300,9 @@ static void io_kill_timeout(struct io_kiocb *req, int status)
        }
 }
 
-static void __io_queue_deferred(struct io_ring_ctx *ctx)
+static void io_queue_deferred(struct io_ring_ctx *ctx)
 {
-       do {
+       while (!list_empty(&ctx->defer_list)) {
                struct io_defer_entry *de = list_first_entry(&ctx->defer_list,
                                                struct io_defer_entry, list);
 
@@ -1310,17 +1311,12 @@ static void __io_queue_deferred(struct io_ring_ctx *ctx)
                list_del_init(&de->list);
                io_req_task_queue(de->req);
                kfree(de);
-       } while (!list_empty(&ctx->defer_list));
+       }
 }
 
 static void io_flush_timeouts(struct io_ring_ctx *ctx)
 {
-       u32 seq;
-
-       if (likely(!ctx->off_timeout_used))
-               return;
-
-       seq = ctx->cached_cq_tail - atomic_read(&ctx->cq_timeouts);
+       u32 seq = ctx->cached_cq_tail - atomic_read(&ctx->cq_timeouts);
 
        while (!list_empty(&ctx->timeout_list)) {
                u32 events_needed, events_got;
@@ -1350,13 +1346,14 @@ static void io_flush_timeouts(struct io_ring_ctx *ctx)
 
 static void io_commit_cqring(struct io_ring_ctx *ctx)
 {
-       io_flush_timeouts(ctx);
-
+       if (unlikely(ctx->off_timeout_used || ctx->drain_used)) {
+               if (ctx->off_timeout_used)
+                       io_flush_timeouts(ctx);
+               if (ctx->drain_used)
+                       io_queue_deferred(ctx);
+       }
        /* order cqe stores with ring update */
        smp_store_release(&ctx->rings->cq.tail, ctx->cached_cq_tail);
-
-       if (unlikely(!list_empty(&ctx->defer_list)))
-               __io_queue_deferred(ctx);
 }
 
 static inline bool io_sqring_full(struct io_ring_ctx *ctx)
@@ -6447,9 +6444,9 @@ static void __io_queue_sqe(struct io_kiocb *req)
                io_queue_linked_timeout(linked_timeout);
 }
 
-static void io_queue_sqe(struct io_kiocb *req)
+static inline void io_queue_sqe(struct io_kiocb *req)
 {
-       if (io_drain_req(req))
+       if (unlikely(req->ctx->drain_used) && io_drain_req(req))
                return;
 
        if (likely(!(req->flags & REQ_F_FORCE_ASYNC))) {
@@ -6573,6 +6570,23 @@ fail_req:
                io_req_complete_failed(req, ret);
                return ret;
        }
+
+       if (unlikely(req->flags & REQ_F_IO_DRAIN)) {
+               ctx->drain_used = true;
+
+               /*
+                * Taking sequential execution of a link, draining both sides
+                * of the link also fullfils IOSQE_IO_DRAIN semantics for all
+                * requests in the link. So, it drains the head and the
+                * next after the link request. The last one is done via
+                * drain_next flag to persist the effect across calls.
+                */
+               if (link->head) {
+                       link->head->flags |= REQ_F_IO_DRAIN;
+                       ctx->drain_next = 1;
+               }
+       }
+
        ret = io_req_prep(req, sqe);
        if (unlikely(ret))
                goto fail_req;
@@ -6591,17 +6605,6 @@ fail_req:
        if (link->head) {
                struct io_kiocb *head = link->head;
 
-               /*
-                * Taking sequential execution of a link, draining both sides
-                * of the link also fullfils IOSQE_IO_DRAIN semantics for all
-                * requests in the link. So, it drains the head and the
-                * next after the link request. The last one is done via
-                * drain_next flag to persist the effect across calls.
-                */
-               if (req->flags & REQ_F_IO_DRAIN) {
-                       head->flags |= REQ_F_IO_DRAIN;
-                       ctx->drain_next = 1;
-               }
                ret = io_req_prep_async(req);
                if (unlikely(ret))
                        goto fail_req;