linux-kernel - Re: [patch 10/12] hrtimer: Determine hard/soft expiry mode for hrtimer sleepers on RT

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Date:   Fri, 26 Jul 2019 16:44:28 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Juergen Gross <jgross@...e.com>
Subject: Re: [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@...utronix.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;