timekeeping: Shorten seq_count region

Shorten the seqcount write hold region to the actual update of the
timekeeper and the related data (e.g vsyscall).

On a contemporary x86 system this reduces the maximum latencies on
Preempt-RT from 8us to 4us on the non-timekeeping cores.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index d20ffda..c4d2a87 100644 (file)
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1341,7 +1341,6 @@ static void update_wall_time(void)
        unsigned long flags;
 
        raw_spin_lock_irqsave(&timekeeper_lock, flags);
-       write_seqcount_begin(&timekeeper_seq);
 
        /* Make sure we're fully resumed: */
        if (unlikely(timekeeping_suspended))
@@ -1393,6 +1392,7 @@ static void update_wall_time(void)
         */
        accumulate_nsecs_to_secs(tk);
 
+       write_seqcount_begin(&timekeeper_seq);
        /* Update clock->cycle_last with the new value */
        clock->cycle_last = tk->cycle_last;
        /*
@@ -1407,9 +1407,8 @@ static void update_wall_time(void)
         */
        memcpy(real_tk, tk, sizeof(*tk));
        timekeeping_update(real_tk, false, false);
-
-out:
        write_seqcount_end(&timekeeper_seq);
+out:
        raw_spin_unlock_irqrestore(&timekeeper_lock, flags);
 }