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From: tip-bot for Sebastian Andrzej Siewior <tipbot@zytor.com>
To: linux-tip-commits@vger.kernel.org
Cc: bigeasy@linutronix.de, rostedt@goodmis.org,
	linux-kernel@vger.kernel.org, peterz@infradead.org,
	tglx@linutronix.de, hpa@zytor.com, mingo@kernel.org
Subject: [tip:timers/core] hrtimer: Determine hard/soft expiry mode for hrtimer sleepers on RT
Date: Tue, 30 Jul 2019 15:16:28 -0700
Message-ID: <tip-a6bc84e64a7dbfdeaae02f434ad1b296f2f9cd1e@git.kernel.org> (raw)
In-Reply-To: <20190726185753.645792403@linutronix.de>

Commit-ID:  a6bc84e64a7dbfdeaae02f434ad1b296f2f9cd1e
Gitweb:     https://git.kernel.org/tip/a6bc84e64a7dbfdeaae02f434ad1b296f2f9cd1e
Author:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
AuthorDate: Fri, 26 Jul 2019 20:30:58 +0200
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Tue, 30 Jul 2019 23:57:57 +0200

hrtimer: Determine hard/soft expiry mode for hrtimer sleepers on RT

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.

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. On expiry 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. These syscall related wakeups are all based on
hrtimer sleepers.

If the current task is in a real-time scheduling class, mark the mode for
hard interrupt expiry.

[ tglx: Split out of a larger combo patch. Added changelog ]

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20190726185753.645792403@linutronix.de

---
 kernel/time/hrtimer.c | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 90dcc4d95e91..c101f88ae8aa 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -1676,6 +1676,16 @@ static enum hrtimer_restart hrtimer_wakeup(struct hrtimer *timer)
 void hrtimer_sleeper_start_expires(struct hrtimer_sleeper *sl,
 				   enum hrtimer_mode mode)
 {
+	/*
+	 * Make the enqueue delivery mode check work on RT. If the sleeper
+	 * was initialized for hard interrupt delivery, force the mode bit.
+	 * This is a special case for hrtimer_sleepers because
+	 * hrtimer_init_sleeper() determines the delivery mode on RT so the
+	 * fiddling with this decision is avoided at the call sites.
+	 */
+	if (IS_ENABLED(CONFIG_PREEMPT_RT) && sl->timer.is_hard)
+		mode |= HRTIMER_MODE_HARD;
+
 	hrtimer_start_expires(&sl->timer, mode);
 }
 EXPORT_SYMBOL_GPL(hrtimer_sleeper_start_expires);
@@ -1683,6 +1693,30 @@ EXPORT_SYMBOL_GPL(hrtimer_sleeper_start_expires);
 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.
+	 *
+	 * 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;

  parent 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
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-bot for Sebastian Andrzej Siewior [this message]
2019-08-01 15:58   ` [tip:timers/core] " 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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