linux-kernel - [tip:timers/core] hrtimer: Move unmarked hrtimers to soft interrupt expiry on RT

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Message-ID: <tip-f5c2f0215e36d76fbb9605283dd7535af09f5770@git.kernel.org>
Date:   Thu, 1 Aug 2019 12:02:36 -0700
From:   tip-bot for Sebastian Andrzej Siewior <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     bigeasy@...utronix.de, hpa@...or.com, linux-kernel@...r.kernel.org,
        mingo@...nel.org, peterz@...radead.org, tglx@...utronix.de
Subject: [tip:timers/core] hrtimer: Move unmarked hrtimers to soft interrupt
 expiry on RT

Commit-ID:  f5c2f0215e36d76fbb9605283dd7535af09f5770
Gitweb:     https://git.kernel.org/tip/f5c2f0215e36d76fbb9605283dd7535af09f5770
Author:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
AuthorDate: Fri, 26 Jul 2019 20:30:57 +0200
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Thu, 1 Aug 2019 20:51:21 +0200

hrtimer: Move unmarked hrtimers to soft interrupt expiry on RT

On PREEMPT_RT not all hrtimers can be expired in hard interrupt context
even if that is perfectly fine on a PREEMPT_RT=n kernel, e.g. because they
take regular spinlocks. Also for latency reasons PREEMPT_RT tries to defer
most hrtimers' expiry into softirq context.

hrtimers marked with HRTIMER_MODE_HARD must be kept in hard interrupt
context expiry mode. Add the required logic.

No functional change for PREEMPT_RT=n kernels.

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

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20190726185753.551967692@linutronix.de



---
 kernel/time/hrtimer.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 0ace301a56f4..90dcc4d95e91 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -1275,8 +1275,17 @@ static void __hrtimer_init(struct hrtimer *timer, clockid_t clock_id,
 			   enum hrtimer_mode mode)
 {
 	bool softtimer = !!(mode & HRTIMER_MODE_SOFT);
-	int base = softtimer ? HRTIMER_MAX_CLOCK_BASES / 2 : 0;
 	struct hrtimer_cpu_base *cpu_base;
+	int base;
+
+	/*
+	 * On PREEMPT_RT enabled kernels hrtimers which are not explicitely
+	 * marked for hard interrupt expiry mode are moved into soft
+	 * interrupt context for latency reasons and because the callbacks
+	 * can invoke functions which might sleep on RT, e.g. spin_lock().
+	 */
+	if (IS_ENABLED(CONFIG_PREEMPT_RT) && !(mode & HRTIMER_MODE_HARD))
+		softtimer = true;
 
 	memset(timer, 0, sizeof(struct hrtimer));
 
@@ -1290,6 +1299,7 @@ static void __hrtimer_init(struct hrtimer *timer, clockid_t clock_id,
 	if (clock_id == CLOCK_REALTIME && mode & HRTIMER_MODE_REL)
 		clock_id = CLOCK_MONOTONIC;
 
+	base = softtimer ? HRTIMER_MAX_CLOCK_BASES / 2 : 0;
 	base += hrtimer_clockid_to_base(clock_id);
 	timer->is_soft = softtimer;
 	timer->is_hard = !softtimer;