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From:   "Ahmed S. Darwish" <a.darwish@linutronix.de>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        "Ahmed S . Darwish" <a.darwish@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>
Subject: [PATCH] sched/clock: expire timer in hardirq context
Date:   Mon,  9 Mar 2020 18:15:29 +0000
Message-Id: <20200309181529.26558-1-a.darwish@linutronix.de>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

To minimize latency, PREEMPT_RT kernels expires hrtimes in preemptible
softirq context by default. This can be overriden by marking the timer's
expiry with HRTIMER_MODE_HARD.

sched_clock_timer is missing this annotation: if its callback is
preempted and the duration of the preemption exceeds the wrap around
time of the underlying clocksource, sched clock will get out of sync.

Mark the sched_clock_timer for expiry in hard interrupt context.

Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
---
 kernel/time/sched_clock.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index e4332e3e2d56..fa3f800d7d76 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -208,7 +208,8 @@ sched_clock_register(u64 (*read)(void), int bits, unsigned long rate)
 
 	if (sched_clock_timer.function != NULL) {
 		/* update timeout for clock wrap */
-		hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL);
+		hrtimer_start(&sched_clock_timer, cd.wrap_kt,
+			      HRTIMER_MODE_REL_HARD);
 	}
 
 	r = rate;
@@ -254,9 +255,9 @@ void __init generic_sched_clock_init(void)
 	 * Start the timer to keep sched_clock() properly updated and
 	 * sets the initial epoch.
 	 */
-	hrtimer_init(&sched_clock_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+	hrtimer_init(&sched_clock_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL_HARD);
 	sched_clock_timer.function = sched_clock_poll;
-	hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL);
+	hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL_HARD);
 }
 
 /*
@@ -293,7 +294,7 @@ void sched_clock_resume(void)
 	struct clock_read_data *rd = &cd.read_data[0];
 
 	rd->epoch_cyc = cd.actual_read_sched_clock();
-	hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL);
+	hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL_HARD);
 	rd->read_sched_clock = cd.actual_read_sched_clock;
 }
 
-- 
2.20.1