[PATCH v2] io_uring: enable toggle of iowait usage when waiting on CQEs

[Jens Axboe <axboe@kernel.dk>]

By default, io_uring marks a waiting task as being in iowait, if it's
sleeping waiting on events and there are pending requests. This isn't
necessarily always useful, and may be confusing on non-storage setups
where iowait isn't expected. It can also cause extra power usage, by
preventing the CPU from entering lower sleep states.

This adds a new enter flag, IORING_ENTER_NO_IOWAIT. If set, then
io_uring will not mark the sleeping task as being in iowait. If the
kernel support this feature, then it will be marked by having the
IORING_FEAT_NO_IOWAIT feature flag set.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

Basic liburing support and a test case here:

https://git.kernel.dk/cgit/liburing/log/?h=iowait

Since v1:
- Add IORING_ENTER_NO_IOWAIT feature flag

diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 050fa8eb2e8f..0d6c83c8d1cf 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -541,6 +541,7 @@ struct io_cqring_offsets {
 #define IORING_ENTER_REGISTERED_RING	(1U << 4)
 #define IORING_ENTER_ABS_TIMER		(1U << 5)
 #define IORING_ENTER_EXT_ARG_REG	(1U << 6)
+#define IORING_ENTER_NO_IOWAIT		(1U << 7)
 
 /*
  * Passed in for io_uring_setup(2). Copied back with updated info on success
@@ -578,6 +579,7 @@ struct io_uring_params {
 #define IORING_FEAT_RECVSEND_BUNDLE	(1U << 14)
 #define IORING_FEAT_MIN_TIMEOUT		(1U << 15)
 #define IORING_FEAT_RW_ATTR		(1U << 16)
+#define IORING_FEAT_NO_IOWAIT		(1U << 17)
 
 /*
  * io_uring_register(2) opcodes and arguments
diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 58003fa6b327..d975e68e91f2 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -2485,8 +2485,18 @@ static int io_cqring_schedule_timeout(struct io_wait_queue *iowq,
 	return READ_ONCE(iowq->hit_timeout) ? -ETIME : 0;
 }
 
+struct ext_arg {
+	size_t argsz;
+	struct timespec64 ts;
+	const sigset_t __user *sig;
+	ktime_t min_time;
+	bool ts_set;
+	bool iowait;
+};
+
 static int __io_cqring_wait_schedule(struct io_ring_ctx *ctx,
 				     struct io_wait_queue *iowq,
+				     struct ext_arg *ext_arg,
 				     ktime_t start_time)
 {
 	int ret = 0;
@@ -2496,7 +2506,7 @@ static int __io_cqring_wait_schedule(struct io_ring_ctx *ctx,
 	 * can take into account that the task is waiting for IO - turns out
 	 * to be important for low QD IO.
 	 */
-	if (current_pending_io())
+	if (ext_arg->iowait && current_pending_io())
 		current->in_iowait = 1;
 	if (iowq->timeout != KTIME_MAX || iowq->min_timeout)
 		ret = io_cqring_schedule_timeout(iowq, ctx->clockid, start_time);
@@ -2509,6 +2519,7 @@ static int __io_cqring_wait_schedule(struct io_ring_ctx *ctx,
 /* If this returns > 0, the caller should retry */
 static inline int io_cqring_wait_schedule(struct io_ring_ctx *ctx,
 					  struct io_wait_queue *iowq,
+					  struct ext_arg *ext_arg,
 					  ktime_t start_time)
 {
 	if (unlikely(READ_ONCE(ctx->check_cq)))
@@ -2522,17 +2533,9 @@ static inline int io_cqring_wait_schedule(struct io_ring_ctx *ctx,
 	if (unlikely(io_should_wake(iowq)))
 		return 0;
 
-	return __io_cqring_wait_schedule(ctx, iowq, start_time);
+	return __io_cqring_wait_schedule(ctx, iowq, ext_arg, start_time);
 }
 
-struct ext_arg {
-	size_t argsz;
-	struct timespec64 ts;
-	const sigset_t __user *sig;
-	ktime_t min_time;
-	bool ts_set;
-};
-
 /*
  * Wait until events become available, if we don't already have some. The
  * application must reap them itself, as they reside on the shared cq ring.
@@ -2610,7 +2613,7 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events, u32 flags,
 							TASK_INTERRUPTIBLE);
 		}
 
-		ret = io_cqring_wait_schedule(ctx, &iowq, start_time);
+		ret = io_cqring_wait_schedule(ctx, &iowq, ext_arg, start_time);
 		__set_current_state(TASK_RUNNING);
 		atomic_set(&ctx->cq_wait_nr, IO_CQ_WAKE_INIT);
 
@@ -3261,6 +3264,8 @@ static int io_get_ext_arg(struct io_ring_ctx *ctx, unsigned flags,
 	const struct io_uring_getevents_arg __user *uarg = argp;
 	struct io_uring_getevents_arg arg;
 
+	ext_arg->iowait = !(flags & IORING_ENTER_NO_IOWAIT);
+
 	/*
 	 * If EXT_ARG isn't set, then we have no timespec and the argp pointer
 	 * is just a pointer to the sigset_t.
@@ -3338,7 +3343,8 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
 			       IORING_ENTER_SQ_WAIT | IORING_ENTER_EXT_ARG |
 			       IORING_ENTER_REGISTERED_RING |
 			       IORING_ENTER_ABS_TIMER |
-			       IORING_ENTER_EXT_ARG_REG)))
+			       IORING_ENTER_EXT_ARG_REG |
+			       IORING_ENTER_NO_IOWAIT)))
 		return -EINVAL;
 
 	/*
@@ -3752,7 +3758,7 @@ static __cold int io_uring_create(unsigned entries, struct io_uring_params *p,
 			IORING_FEAT_RSRC_TAGS | IORING_FEAT_CQE_SKIP |
 			IORING_FEAT_LINKED_FILE | IORING_FEAT_REG_REG_RING |
 			IORING_FEAT_RECVSEND_BUNDLE | IORING_FEAT_MIN_TIMEOUT |
-			IORING_FEAT_RW_ATTR;
+			IORING_FEAT_RW_ATTR | IORING_FEAT_NO_IOWAIT;
 
 	if (copy_to_user(params, p, sizeof(*p))) {
 		ret = -EFAULT;
-- 
Jens Axboe