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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Austin Schuh <austin.linux@gmail.com>
Cc: axboe@kernel.dk, io-uring@vger.kernel.org,
	linux-rt-devel@lists.linux.dev, clrkwllms@kernel.org,
	rostedt@goodmis.org, tglx@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] io_uring/wait: wake real-time waiters from hard
 interrupt context
Message-ID: <20261017151945.q-LijyqZ@linutronix.de>
References: <20261017013303.3919170-1-austin.linux@gmail.com>
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In-Reply-To: <20261017013303.3919170-1-austin.linux@gmail.com>

On 2026-10-16 18:33:03 [-0700], Austin Schuh wrote:
> --- a/io_uring/wait.c
> +++ b/io_uring/wait.c
> @@ -120,23 +121,39 @@ static enum hrtimer_restart io_cqring_min_timer_wakeup(struct hrtimer *timer)
>  	return io_cqring_timer_wakeup(timer);
>  }
>  
> +/*
> + * On PREEMPT_RT, hrtimers not marked for hard interrupt expiry expire in the
> + * ktimers thread, at the lowest real-time priority, so a real-time waiter
> + * isn't woken until every runnable real-time task on its CPU has blocked.
> + * Like hrtimer sleepers (see __hrtimer_setup_sleeper()), expire the timer in
> + * hard interrupt context if the waiter is a real-time task. Both callbacks
> + * only look at the ring and wake the task, which is safe there.

Please simply refer to __hrtimer_setup_sleeper() for the policy.

> + */
> +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;
> +}

Sebastian