[PATCH 12/12] io_uring: optimise non-drain path

Pavel Begunkov
2021-06-14T22:38:21Z

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>
---
fs/io_uring.c | 57 +++++++++++++++++++++++++++------------------------
1 file changed, 30 insertions(+), 27 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 29b705201ca3..5828ffdbea82 100644
--- 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 @@ static int io_submit_sqe(struct io_ring_ctx *ctx, struct io_kiocb *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 @@ static int io_submit_sqe(struct io_ring_ctx *ctx, struct io_kiocb *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;
--
2.31.1