io_uring/wait: wake real-time waiters from hard interrupt context

Since commit cebf123c634a ("io_uring: implement our own schedule timeout
handling"), a CQ wait with a timeout sleeps on io_uring's own hrtimer
instead of an hrtimer sleeper.  On PREEMPT_RT, an hrtimer that isn't
marked for hard interrupt expiry expires in the per-CPU ktimers thread,
which runs at SCHED_FIFO 1.  A real-time task waiting in io_uring_enter()
is therefore not woken at its deadline while any other real-time task is
runnable on its CPU: the wakeup waits until every one of them has blocked.

hrtimer sleepers, which schedule_hrtimeout_range_clock() used to give
io_uring, don't have this problem: __hrtimer_setup_sleeper() marks the
timer hard when the sleeping task is in a real-time or deadline class.
Both of io_uring's callbacks are fine in hard interrupt context:
io_cqring_timer_wakeup() sets a flag and calls wake_up_process(), and
io_cqring_min_timer_wakeup() only reads ring state, under RCU, before
doing the same or re-arming the timer.  Apply the sleeper rule to them.

Measured on a Jetson Orin Nano running 6.18.39-rt with this change
backported: a SCHED_FIFO 40 task waits in io_uring_enter() for 40,000
deadlines 500us apart, while a SCHED_FIFO 20 task runs bursts of up to
1ms on the same CPU.  Before, 6,647 wakeups were more than 100us late,
the latest by 1,040us, and 2,758 deadlines passed without a wakeup at
all.  After, none were more than 100us late and the latest was 18us.

This mirrors epoll: epoll_pwait2() waits on an hrtimer sleeper, so its
timeout is already hard for real-time tasks, and on the same setup it
was never more than 17us late.

Fixes: cebf123c634a ("io_uring: implement our own schedule timeout handling")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
Link: https://patch.msgid.link/20261017171726.601443-1-austin.linux@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

0 files changed, 0 insertions, 0 deletions