linux-kernel - Re: [PATCH 07/10] sched: Migrate sched to use new tick dependency mask model

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Date:	Mon, 10 Aug 2015 16:16:58 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Juri Lelli <juri.lelli@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	Ingo Molnar <mingo@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 07/10] sched: Migrate sched to use new tick dependency
 mask model

On Mon, Aug 10, 2015 at 03:02:04PM +0100, Juri Lelli wrote:
> Hi,
> 
> On 04/08/15 08:41, Peter Zijlstra wrote:
> > On Mon, Aug 03, 2015 at 07:30:32PM +0200, Frederic Weisbecker wrote:
> >>> But you've forgotten about SCHED_DEADLINE, we count those in:
> >>> rq->dl.dl_nr_running.
> >>
> >> Indeed. Hmm, there is no preemption between SCHED_DEALINE tasks, right?
> >> So I can treat it like SCHED_FIFO.
> > 
> > Sadly no. Even though EDF has static job priority (once a job is
> > activated its priority doesn't change anymore) DEADLINE also has a CBS
> > component and that needs the tick regardless, even with a single task.
> > 
> > So any deadline task running means we cannot stop the tick.
> 
> Well, couldn't we stop it when we use hrtick? As start_hrtick_dl() sets
> the hrtick to fire at tasks runtime depletion instant.

Hrtick() only does the task tick part of scheduler_tick(). There are
many other things that need to be updated. Cpu load active (which is buggy
with full dynticks btw. because __update_cpu_load() expects nothing else than
regular frequency updates or idle decay. There is also calc_global_load(),
load balancing stuffs.

I considered many times relying on hrtick btw but everyone seem to say it has a lot
of overhead, especially due to clock reprogramming on schedule() calls.
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