On Wed, 2017-04-05 at 23:36 -0700, Wanpeng Li wrote: > This patch offsets the tick to avert all ticks alignment in order > that the vtime sampling does not end up "in phase" with the jiffies > incrementing. > > Reported-by: Luiz Capitulino <[email protected]> > Suggested-by: Rik van Riel <[email protected]> > Cc: Frederic Weisbecker <[email protected]> > Cc: Rik van Riel <[email protected]> > Cc: Mike Galbraith <[email protected]> > Cc: Luiz Capitulino <[email protected]> > Cc: Thomas Gleixner <[email protected]> > Cc: Peter Zijlstra <[email protected]> > Signed-off-by: Wanpeng Li <[email protected]>
Reviewed-by: Rik van Riel <[email protected]> > +++ b/kernel/time/tick-sched.c > @@ -1197,8 +1197,12 @@ void tick_setup_sched_timer(void) > /* Get the next period (per-CPU) */ > hrtimer_set_expires(&ts->sched_timer, > tick_init_jiffy_update()); > > - /* Offset the tick to avert jiffies_lock contention. */ > - if (sched_skew_tick) { > + /* > + * Offset the tick to avert jiffies_lock contention, and all > ticks > + * alignment in order that the vtime sampling does not end > up "in > + * phase" with the jiffies incrementing. > + */ I feel like part of the explanation is missing from this comment, but I am not sure how to make it better without making it way too long :) > + if (sched_skew_tick || tick_nohz_full_enabled()) { > u64 offset = ktime_to_ns(tick_period) >> 1; > do_div(offset, num_possible_cpus()); > offset *= smp_processor_id();
