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SubjectRe: [patch 10/12] hrtimer: Determine hard/soft expiry mode for hrtimer sleepers on RT
On Fri, 26 Jul 2019 20:30:58 +0200
Thomas Gleixner <tglx@linutronix.de> wrote:

> +++ b/kernel/time/hrtimer.c
> @@ -1662,6 +1662,30 @@ static enum hrtimer_restart hrtimer_wake
> static void __hrtimer_init_sleeper(struct hrtimer_sleeper *sl,
> clockid_t clock_id, enum hrtimer_mode mode)
> {
> + /*
> + * On PREEMPT_RT enabled kernels hrtimers which are not explicitely
> + * marked for hard interrupt expiry mode are moved into soft
> + * interrupt context either for latency reasons or because the
> + * hrtimer callback takes regular spinlocks or invokes other
> + * functions which are not suitable for hard interrupt context on
> + * PREEMPT_RT.

Have we marked all timer handlers that have normal spin_locks as
HRTIMER_MODE_SOFT? Otherwise, can't we switch one to hard below and
having their handler grab a spin_lock/mutex in hard interrupt context
in RT?

-- Steve


> + *
> + * The hrtimer_sleeper callback is RT compatible in hard interrupt
> + * context, but there is a latency concern: Untrusted userspace can
> + * spawn many threads which arm timers for the same expiry time on
> + * the same CPU. That causes a latency spike due to the wakeup of
> + * a gazillion threads.
> + *
> + * OTOH, priviledged real-time user space applications rely on the
> + * low latency of hard interrupt wakeups. If the current task is in
> + * a real-time scheduling class, mark the mode for hard interrupt
> + * expiry.
> + */
> + if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> + if (task_is_realtime(current) && !(mode & HRTIMER_MODE_SOFT))
> + mode |= HRTIMER_MODE_HARD;
> + }
> +
> __hrtimer_init(&sl->timer, clock_id, mode);
> sl->timer.function = hrtimer_wakeup;
> sl->task = current;

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