Date: Mon, 10 Aug 2015 16:16:58 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Juri Lelli <juri.lelli@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>, LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Preeti U Murthy <preeti@linux.vnet.ibm.com>,
        Christoph Lameter <cl@linux.com>, Ingo Molnar <mingo@kernel.org>,
        Viresh Kumar <viresh.kumar@linaro.org>, Rik van Riel <riel@redhat.com>
Subject: Re: [PATCH 07/10] sched: Migrate sched to use new tick dependency
 mask model
Message-ID: <20150810141656.GA31251@lerouge>
References: <1437669735-8786-1-git-send-email-fweisbec@gmail.com>
 <1437669735-8786-8-git-send-email-fweisbec@gmail.com>
 <20150803140046.GK19282@twins.programming.kicks-ass.net>
 <20150803145031.GD25554@lerouge>
 <20150803170911.GV25159@twins.programming.kicks-ass.net>
 <20150803173031.GB26022@lerouge>
 <20150804074116.GH25159@twins.programming.kicks-ass.net>
 <55C8AEDC.90603@arm.com>
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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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