[PATCH 5/6] io_uring: use TWA_SIGNAL_NO_IPI if IORING_SETUP_COOP_TASKRUN is used

Jens Axboe
2022-04-26T01:49:17Z

If this is set, io_uring will never use an IPI to deliver a task_work
notification. This can be used in the common case where a single task or
thread communicates with the ring, and doesn't rely on
io_uring_cqe_peek().

This provides a noticeable win in performance, both from eliminating
the IPI itself, but also from avoiding interrupting the submitting
task unnecessarily.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
fs/io_uring.c | 17 +++++++++++++----
include/uapi/linux/io_uring.h | 8 ++++++++
2 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 7e9ac5fd3a8c..5e4842cd21c2 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -11696,12 +11696,20 @@ static __cold int io_uring_create(unsigned entries, struct io_uring_params *p,
ctx->user = get_uid(current_user());

/*
- * For SQPOLL, we just need a wakeup, always.
+ * For SQPOLL, we just need a wakeup, always. For !SQPOLL, if
+ * COOP_TASKRUN is set, then IPIs are never needed by the app.
*/
- if (ctx->flags & IORING_SETUP_SQPOLL)
+ ret = -EINVAL;
+ if (ctx->flags & IORING_SETUP_SQPOLL) {
+ /* IPI related flags don't make sense with SQPOLL */
+ if (ctx->flags & IORING_SETUP_COOP_TASKRUN)
+ goto err;
ctx->notify_method = TWA_SIGNAL_NO_IPI;
- else
+ } else if (ctx->flags & IORING_SETUP_COOP_TASKRUN) {
+ ctx->notify_method = TWA_SIGNAL_NO_IPI;
+ } else {
ctx->notify_method = TWA_SIGNAL;
+ }

/*
* This is just grabbed for accounting purposes. When a process exits,
@@ -11800,7 +11808,8 @@ static long io_uring_setup(u32 entries, struct io_uring_params __user *params)
if (p.flags & ~(IORING_SETUP_IOPOLL | IORING_SETUP_SQPOLL |
IORING_SETUP_SQ_AFF | IORING_SETUP_CQSIZE |
IORING_SETUP_CLAMP | IORING_SETUP_ATTACH_WQ |
- IORING_SETUP_R_DISABLED | IORING_SETUP_SUBMIT_ALL))
+ IORING_SETUP_R_DISABLED | IORING_SETUP_SUBMIT_ALL |
+ IORING_SETUP_COOP_TASKRUN))
return -EINVAL;

return io_uring_create(entries, &p, params);
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 5fb52bf32435..4654842ace88 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -104,6 +104,14 @@ enum {
#define IORING_SETUP_ATTACH_WQ (1U << 5) /* attach to existing wq */
#define IORING_SETUP_R_DISABLED (1U << 6) /* start with ring disabled */
#define IORING_SETUP_SUBMIT_ALL (1U << 7) /* continue submit on error */
+/*
+ * Cooperative task running. When requests complete, they often require
+ * forcing the submitter to transition to the kernel to complete. If this
+ * flag is set, work will be done when the task transitions anyway, rather
+ * than force an inter-processor interrupt reschedule. This avoids interrupting
+ * a task running in userspace, and saves an IPI.
+ */
+#define IORING_SETUP_COOP_TASKRUN (1U << 8)

enum {
IORING_OP_NOP,
--
2.35.1