io_uring: enable toggle of iowait usage when waiting on CQEs

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 account the sleeping task as being in iowait. If the
kernel supports this feature, then it will be marked by having the
IORING_FEAT_NO_IOWAIT feature flag set.

As the kernel currently does not support separating the iowait
accounting and CPU frequency boosting, the IORING_ENTER_NO_IOWAIT
controls both of these at the same time. In the future, if those do end
up being split, then it'd be possible to control them separately.
However, it seems more likely that the kernel will decouple iowait and
CPU frequency boosting anyway.

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

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