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

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Since commit cebf123c63 ("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: cebf123c63 ("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>
This commit is contained in:
Austin Schuh 2026-10-17 10:17:26 -07:00 committed by Jens Axboe
parent c82621eb2c
commit 5b01d5955a
1 changed files with 13 additions and 3 deletions
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16
io_uring/wait.c
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@ -3,6 +3,7 @@
* Waiting for completion events
*/
#include <linux/kernel.h>
#include <linux/sched/rt.h>
#include <linux/sched/signal.h>
#include <linux/io_uring.h>
#include <linux/time_namespace.h>
@ -120,23 +121,32 @@ static enum hrtimer_restart io_cqring_min_timer_wakeup(struct hrtimer *timer)
return io_cqring_timer_wakeup(timer);
}
/* Same policy as for hrtimer sleepers, see __hrtimer_setup_sleeper(). */
static enum hrtimer_mode io_cqring_timer_mode(void)
{
if (IS_ENABLED(CONFIG_PREEMPT_RT) && rt_or_dl_task_policy(current))
return HRTIMER_MODE_ABS_HARD;
return HRTIMER_MODE_ABS;
}
static int io_cqring_schedule_timeout(struct io_wait_queue *iowq,
clockid_t clock_id, ktime_t start_time)
{
enum hrtimer_mode mode = io_cqring_timer_mode();
ktime_t timeout;
if (iowq->min_timeout) {
timeout = ktime_add_ns(iowq->min_timeout, start_time);
hrtimer_setup_on_stack(&iowq->t, io_cqring_min_timer_wakeup, clock_id,
HRTIMER_MODE_ABS);
mode);
} else {
timeout = iowq->timeout;
hrtimer_setup_on_stack(&iowq->t, io_cqring_timer_wakeup, clock_id,
HRTIMER_MODE_ABS);
mode);
}
hrtimer_set_expires_range_ns(&iowq->t, timeout, 0);
hrtimer_start_expires(&iowq->t, HRTIMER_MODE_ABS);
hrtimer_start_expires(&iowq->t, mode);
if (!READ_ONCE(iowq->hit_timeout))
schedule();