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Date: Tue, 19 Jan 2016 19:56:47 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Byungchul Park <byungchul.park@lge.com>,
        Chris Metcalf <cmetcalf@ezchip.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Luiz Capitulino <lcapitulino@redhat.com>,
        Christoph Lameter <cl@linux.com>,
        "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
        Mike Galbraith <efault@gmx.de>, Rik van Riel <riel@redhat.com>
Subject: Re: [PATCH 1/4] sched: Don't account tickless CPU load on tick
Message-ID: <20160119185647.GA6357@twins.programming.kicks-ass.net>
References: <1452700891-21807-1-git-send-email-fweisbec@gmail.com>
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 <20160119130857.GC6344@twins.programming.kicks-ass.net>
 <20160119162210.GA5317@lerouge>
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On Tue, Jan 19, 2016 at 05:22:11PM +0100, Frederic Weisbecker wrote:
> On Tue, Jan 19, 2016 at 02:08:57PM +0100, Peter Zijlstra wrote:
> > On Wed, Jan 13, 2016 at 05:01:28PM +0100, Frederic Weisbecker wrote:
> > > The cpu load update on tick doesn't care about dynticks and as such is
> > > buggy when occuring on nohz ticks (including idle ticks) as it resets
> > > the jiffies snapshot that was recorded on nohz entry. We eventually
> > > ignore the potentially long tickless load that happened before the
> > > tick.
> > 
> > I don't get it, how can we call scheduler_tick() while
> > tick_nohz_tick_stopped() ?
> 
> tick_nohz_tick_stopped() (which is ts->tick_stopped == 1) doesn't actually
> mean that the tick is really stopped. It just means that the tick fires only
> when it's really needed (timer list expired, RCU stuff, irq_work, ...).

That's insane and broken. Fix _that_.

If RCU, irq_work etc.. needs the tick, do not stop the tick.