linux-kernel - Re: [PATCH 2/3] sched: Correctly handle nohz ticks cpu load accounting

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Message-ID: <CANRm+CyZtgAXEvnX76Jh5uJwvbYuJnKrkzUXa4jFKnBLSvtb1Q@mail.gmail.com>
Date:	Wed, 20 Apr 2016 15:59:43 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Byungchul Park <byungchul.park@....com>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>, Rik van Riel <riel@...hat.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 2/3] sched: Correctly handle nohz ticks cpu load accounting

Hi Frederic,
2016-04-13 21:56 GMT+08:00 Frederic Weisbecker <fweisbec@...il.com>:
> Ticks can happen while the CPU is in dynticks-idle or dynticks-singletask
> mode. In fact "nohz" or "dynticks" only mean that we exit the periodic
> mode and we try to minimize the ticks as much as possible. The nohz
> subsystem uses a confusing terminology with the internal state
> "ts->tick_stopped" which is also available through its public interface
> with tick_nohz_tick_stopped(). This is a misnomer as the tick is instead
> reduced with the best effort rather than stopped. In the best case the
> tick can indeed be actually stopped but there is no guarantee about that.
> If a timer needs to fire one second later, a tick will fire while the
> CPU is in nohz mode and this is a very common scenario.
>
> Now this confusion happens to be a problem with cpu load updates:
> cpu_load_update_active() doesn't handle nohz ticks correctly because it
> assumes that ticks are completely stopped in nohz mode and that
> cpu_load_update_active() can't be called in dynticks mode. When that
> happens, the whole previous tickless load is ignored and the function
> just records the load for the current tick, ignoring potentially long
> idle periods behind.

This is for one timer interrupt per second scenario, right?

Regards,
Wanpeng Li