Re: [Query]: tick-sched: why don't we stop tick when we are running idle task?

From: Frederic Weisbecker <fweisbec@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Viresh Kumar <viresh.kumar@linaro.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Lists linaro-kernel <linaro-kernel@lists.linaro.org>
Subject: Re: [Query]: tick-sched: why don't we stop tick when we are running idle task?
Date: Tue, 15 Apr 2014 11:30:04 +0200	[thread overview]
Message-ID: <20140415093002.GL1877@localhost.localdomain> (raw)
In-Reply-To: <20140414120600.GJ11096@twins.programming.kicks-ass.net>

On Mon, Apr 14, 2014 at 02:06:00PM +0200, Peter Zijlstra wrote:
> On Mon, Apr 14, 2014 at 05:22:30PM +0530, Viresh Kumar wrote:
> > On 14 April 2014 17:17, Peter Zijlstra <peterz@infradead.org> wrote:
> > > What causes this tick? I was under the impression that once there's a
> > > single task (not doing any syscalls) and the above issues are sorted, no
> > > more tick would happen.
> > 
> > This is what Frederic told me earlier:
> > 
> > https://lkml.org/lkml/2014/2/13/238
> 
> That's a bit of a non-answer. I'm fairly sure its not a gazillion
> issues, since the actual scheduler tick doesn't actually do that much.
> 
> So start by enumerating what is actually required.

Ok, I'm a bit buzy with a conference right now but I'm going to summarize that
soonish.

> 
> The 2), which I suppose you're now trying to implement is I think
> entirely the wrong way. The tick really assumes it runs local, moving it
> to another CPU is insane.

There is probably a few things that assume local calls but last time
I checked I had the impression that it was fairly possible to call sched_class::task_tick()
remotely. rq is locked, no reference to "current", use rq accessors...

OTOH scheduler_tick() itself definetly requires local calls.