time/sched_clock: Expire timer in hardirq context

To minimize latency, PREEMPT_RT kernels expires hrtimers 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>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200309181529.26558-1-a.darwish@linutronix.de

diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index e4332e3e2d5691430314ba70911a8610e7044d79..fa3f800d7d763feac3fcdebc99939f2ef749a23b 100644 (file)
--- 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;
 }