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    Subject[PATCH 5.5 050/257] time/sched_clock: Expire timer in hardirq context
    Date
    From: Ahmed S. Darwish <a.darwish@linutronix.de>

    [ Upstream commit 2c8bd58812ee3dbf0d78b566822f7eacd34bdd7b ]

    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
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    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 dbd69052eaa66..a5538dd76a819 100644
    --- a/kernel/time/sched_clock.c
    +++ b/kernel/time/sched_clock.c
    @@ -207,7 +207,8 @@ 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;
    @@ -251,9 +252,9 @@ 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);
    }

    /*
    @@ -290,7 +291,7 @@ 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


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