| Date | Fri, 26 Jul 2019 16:44:28 -0400 | From | Steven Rostedt <> | Subject | Re: [patch 10/12] hrtimer: Determine hard/soft expiry mode for hrtimer sleepers on RT |
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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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