Re: [PATCH] tick/nohz: Fix wrong user and system time accouting against vtime sampling

[Thomas Gleixner]

On Wed, 5 Apr 2017, Wanpeng Li wrote:
> +     /*
> +      * 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.
> +      */
> +     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();

That's not a fix, that's just papering over the problem.

       offset = 1ms / 2 = 500us = 500000ns;
       offset /= 144 = 3472ns

So CPU0 and CPU1 ticks are ~3 microseconds apart. That merily reduces the
probability of the issue, but does not prevent it.

Thanks,

        tglx