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From: Thomas Gleixner <tglx@linutronix.de>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	x86@kernel.org, Sebastian Siewior <bigeasy@linutronix.de>,
	Anna-Maria Gleixner <anna-maria@linutronix.de>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Jiri Olsa <jolsa@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>,
	Juergen Gross <jgross@suse.com>
Subject: Re: [patch 10/12] hrtimer: Determine hard/soft expiry mode for hrtimer sleepers on RT
Date: Fri, 26 Jul 2019 22:52:18 +0200 (CEST)
Message-ID: <alpine.DEB.2.21.1907262249210.1791@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20190726164428.40a4e4b4@gandalf.local.home>

On Fri, 26 Jul 2019, Steven Rostedt wrote:

> 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?

See patch 09/12. We move all timers into soft mode which are not marked
MODE_HARD.

> > +	 *
> > +	 * 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;

It's the wakeup function and nothing is supposed to override that.

Thanks,

	tglx

  reply index

Thread overview: 61+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-26 18:30 [patch 00/12] (hr)timers: Prepare for PREEMPT_RT support Thomas Gleixner
2019-07-26 18:30 ` [patch 01/12] hrtimer: Remove task argument from hrtimer_init_sleeper() Thomas Gleixner
2019-07-26 19:57   ` Steven Rostedt
2019-07-26 20:01     ` Thomas Gleixner
2019-07-30 22:07   ` [tip:timers/core] " tip-bot for Thomas Gleixner
2019-07-26 18:30 ` [patch 02/12] hrtimer: Consolidate hrtimer_init() + hrtimer_init_sleeper() calls Thomas Gleixner
2019-07-30 22:08   ` [tip:timers/core] " tip-bot for Sebastian Andrzej Siewior
2019-08-01 15:49   ` tip-bot for Sebastian Andrzej Siewior
2019-07-26 18:30 ` [patch 03/12] hrtimer: Introduce HARD expiry mode Thomas Gleixner
2019-07-30 22:10   ` [tip:timers/core] " tip-bot for Sebastian Andrzej Siewior
2019-08-01 15:52   ` tip-bot for Sebastian Andrzej Siewior
2019-07-26 18:30 ` [patch 04/12] sched: Mark hrtimers to expire in hard interrupt context Thomas Gleixner
2019-07-30 22:11   ` [tip:timers/core] " tip-bot for Thomas Gleixner
2019-08-01 15:53   ` tip-bot for Sebastian Andrzej Siewior
2019-08-01 18:58   ` tip-bot for Sebastian Andrzej Siewior
2019-07-26 18:30 ` [patch 05/12] perf/core: " Thomas Gleixner
2019-07-30 22:12   ` [tip:timers/core] " tip-bot for Thomas Gleixner
2019-08-01 15:54   ` tip-bot for Sebastian Andrzej Siewior
2019-08-01 18:59   ` tip-bot for Sebastian Andrzej Siewior
2019-07-26 18:30 ` [patch 06/12] watchdog: Mark watchdog_hrtimer " Thomas Gleixner
2019-07-30 22:13   ` [tip:timers/core] " tip-bot for Sebastian Andrzej Siewior
2019-08-01 15:55   ` tip-bot for Sebastian Andrzej Siewior
2019-08-01 19:00   ` tip-bot for Sebastian Andrzej Siewior
2019-07-26 18:30 ` [patch 07/12] KVM: LAPIC: Mark hrtimer " Thomas Gleixner
2019-07-26 19:41   ` Paolo Bonzini
2019-07-30 22:14   ` [tip:timers/core] " tip-bot for Sebastian Andrzej Siewior
2019-08-01 15:55   ` tip-bot for Sebastian Andrzej Siewior
2019-08-01 19:01   ` tip-bot for Sebastian Andrzej Siewior
2019-07-26 18:30 ` [patch 08/12] tick: Mark tick related hrtimers to expiry " Thomas Gleixner
2019-07-30 22:14   ` [tip:timers/core] " tip-bot for Sebastian Andrzej Siewior
2019-08-01 15:56   ` tip-bot for Sebastian Andrzej Siewior
2019-08-01 19:01   ` tip-bot for Sebastian Andrzej Siewior
2019-07-26 18:30 ` [patch 09/12] hrtimer: Move unmarked hrtimers to soft interrupt expiry on RT Thomas Gleixner
2019-07-30 22:15   ` [tip:timers/core] " tip-bot for Sebastian Andrzej Siewior
2019-08-01 15:57   ` tip-bot for Sebastian Andrzej Siewior
2019-08-01 19:02   ` tip-bot for Sebastian Andrzej Siewior
2019-07-26 18:30 ` [patch 10/12] hrtimer: Determine hard/soft expiry mode for hrtimer sleepers " Thomas Gleixner
2019-07-26 20:44   ` Steven Rostedt
2019-07-26 20:52     ` Thomas Gleixner [this message]
2019-07-26 20:56       ` Steven Rostedt
2019-07-26 21:16   ` Julia Cartwright
2019-07-26 21:30     ` Steven Rostedt
2019-07-26 21:35     ` Thomas Gleixner
2019-07-30 22:16   ` [tip:timers/core] " tip-bot for Sebastian Andrzej Siewior
2019-08-01 15:58   ` tip-bot for Sebastian Andrzej Siewior
2019-08-01 19:03   ` tip-bot for Sebastian Andrzej Siewior
2019-07-26 18:30 ` [patch 11/12] hrtimer: Prepare support for PREEMPT_RT Thomas Gleixner
2019-07-28  9:06   ` Juergen Gross
2019-07-29 15:08     ` Steven Rostedt
2019-07-29 17:30       ` Paolo Bonzini
2019-07-31  8:45         ` Juergen Gross
2019-07-30 22:17   ` [tip:timers/core] " tip-bot for Anna-Maria Gleixner
2019-08-01 15:58   ` tip-bot for Anna-Maria Gleixner
2019-08-01 19:04   ` tip-bot for Anna-Maria Gleixner
2019-08-20 13:26     ` Frederic Weisbecker
2019-08-23  2:12       ` [tip: timers/core] hrtimer: Improve comments on handling priority inversion against softirq kthread tip-bot2 for Frederic Weisbecker
2019-07-26 18:31 ` [patch 12/12] timers: Prepare support for PREEMPT_RT Thomas Gleixner
2019-07-30 22:17   ` [tip:timers/core] " tip-bot for Anna-Maria Gleixner
2019-08-01 15:59   ` tip-bot for Anna-Maria Gleixner
2019-08-01 19:04   ` tip-bot for Anna-Maria Gleixner
2019-07-29 19:45 ` [patch 00/12] (hr)timers: Prepare for PREEMPT_RT support Peter Zijlstra

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