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From: Thomas Gleixner <tglx@linutronix.de>
To: Wanpeng Li <kernellwp@gmail.com>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Wanpeng Li <wanpeng.li@hotmail.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Rik van Riel <riel@redhat.com>, Mike Galbraith <efault@gmx.de>,
	Luiz Capitulino <lcapitulino@redhat.com>
Subject: Re: [PATCH] tick/nohz: Fix wrong user and system time accouting against vtime sampling
Date: Mon, 10 Apr 2017 17:45:56 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.20.1704101738240.2906@nanos> (raw)
In-Reply-To: <1491460584-4908-1-git-send-email-wanpeng.li@hotmail.com>

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

  parent reply	other threads:[~2017-04-10 15:46 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-04-06  6:36 [PATCH] tick/nohz: Fix wrong user and system time accouting against vtime sampling Wanpeng Li
2017-04-06 14:36 ` Rik van Riel
2017-04-07 16:58 ` Luiz Capitulino
2017-04-10 15:45 ` Thomas Gleixner [this message]
2017-04-10 18:01   ` Frederic Weisbecker
2017-04-10 21:51     ` Wanpeng Li

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