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

From: Peter Zijlstra <peterz@infradead.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
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: Thu, 22 May 2014 10:44:19 +0200	[thread overview]
Message-ID: <20140522084419.GS30445@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20140513233036.GH13828@localhost.localdomain>

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On Wed, May 14, 2014 at 01:30:39AM +0200, Frederic Weisbecker wrote:
> On Fri, May 09, 2014 at 02:14:10PM +0530, Viresh Kumar wrote:
> > On 23 April 2014 16:42, Viresh Kumar <viresh.kumar@linaro.org> wrote:
> > > On 15 April 2014 15:00, Frederic Weisbecker <fweisbec@gmail.com> wrote:
> > >> Ok, I'm a bit buzy with a conference right now but I'm going to summarize that
> > >> soonish.
> > 
> > Hi Frederic,
> > 
> > Please see if you can find some time to close this, that would be very
> > helpful :)
> > 
> > Thanks
> 
> I'm generally worried about the accounting in update_curr() that periodically
> updates stats. I have no idea which of these stats could be read by other CPUs:
> vruntime, load bandwitdth, etc...

update_curr() principally does the sum_exec_runtime and vruntime. Now
vruntime is only interesting for other cpus when moving tasks across
CPUs, so see below on load-balancing.

sum_exec_runtime is used for a number of task stats, but when nobody
looks at those it shouldn't matter.

So rather than constantly force update them for no purpose, update them
on-demand. So when someone reads those cputime stats, prod the task/cpu.
I think you can do a remote update_curr() just fine.

And I suppose you also need to do something with task_tick_numa(), that
relies on the tick regardless of nr_running. And that's very much not
something you can do remotely. As it stands I think the numa balancing
and nohz_full are incompatible.

> Also without tick:
> 
> * we don't poll anymore on trigger_load_balance()
> 
> * __update_cpu_load() requires fixed rate periodic polling. Alex Shi had
> patches for that but I'm not sure if that's going to be merged
> 
> * rq->rt_avg accounting?

So I think typically we don't want load-balancing to happen when we're
on a nohz_full cpu and there's only the one task running.

So what you can do is extend the existing nohz balancing (which
currently only deals with CPU_IDLE) to also remote balance CPU_NOT_IDLE
when nr_running == 1.

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