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SubjectRe: [PATCH] tick/nohz: Fix wrong user and system time accouting against vtime sampling
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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 <lcapitulino@redhat.com>
> Suggested-by: Rik van Riel <riel@redhat.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Mike Galbraith <efault@gmx.de>
> Cc: Luiz Capitulino <lcapitulino@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>

Reviewed-by: Rik van Riel <riel@redhat.com>

> +++ 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();

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