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

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Date:   Mon, 10 Apr 2017 17:45:56 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Wanpeng Li <kernellwp@...il.com>
cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Rik van Riel <riel@...hat.com>,
        Mike Galbraith <efault@....de>,
        Luiz Capitulino <lcapitulino@...hat.com>
Subject: Re: [PATCH] tick/nohz: Fix wrong user and system time accouting
 against vtime sampling

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