Date: Tue, 15 Dec 2015 22:24:24 +0100
From: Luca Abeni <luca.abeni@unitn.it>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Vincent Guittot <vincent.guittot@linaro.org>,
        Steve Muckle <steve.muckle@linaro.org>, Ingo Molnar <mingo@redhat.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        "linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Juri Lelli <Juri.Lelli@arm.com>,
        Patrick Bellasi <patrick.bellasi@arm.com>,
        Michael Turquette <mturquette@baylibre.com>
Subject: Re: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in
 scale_rt_capacity
Message-ID: <20151215222424.32f4de89@luca-1225C>
In-Reply-To: <20151215134229.GH6357@twins.programming.kicks-ass.net>
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On Tue, 15 Dec 2015 14:42:29 +0100
Peter Zijlstra <peterz@infradead.org> wrote:
> On Tue, Dec 15, 2015 at 02:30:07PM +0100, Luca Abeni wrote:
> 
> > >So I remember something else from the BFQ code, which also had to
> > >track entries for the 0-lag stuff, and I just had a quick peek at
> > >that code again. And what they appear to do is keep inactive
> > >entries with a lag deficit in a separate tree (the idle tree).
> > >
> > >And every time they update the vtime, they also push fwd the idle
> > >tree and expire entries on that.
> > I am not sure if I understand correctly the idea (I do not know the
> > BFQ code; I'll have a look), but I think I tried something similar:
> > - When a task blocks, instead of arming the inactive timer I can
> > insert the task in an "active non contending" tree (to use GRUB
> > terminology)
> > - So, when some sched deadline function is invoked, I check the
> > "0-lag time" of the first task in the "active non contending" tree,
> > and if that time is passed I remove the task from the tree and
> > adjust the active utilisation
> > 
> > The resulting code ended up being more complex (basically, I needed
> > to handle the "active non contending" tree and to check it in
> > task_tick_dl() and update_curr_dl()). But maybe I did it wrong...
> > I'll try this approach again, after looking ad the BFQ code.
> 
> That sounds about right.
> 
> I've no idea if its more or less work. I just had vague memories on an
> alternative approach to the timer.
> 
> Feel free to stick with the timer if that works better, just wanted to
> mention there are indeed alternative solutions.
Ok; I'll try to implement this alternative approach again, after
looking at BFQ, to see if it turns out to be simpler or more complex
than the timer-based approach.

If there is interest, I'll send an RFC with these patches after some
testing.


			Thanks,
				Luca
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