linux-kernel - Re: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in scale_rt

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Message-ID: <20151215134229.GH6357@twins.programming.kicks-ass.net>
Date:	Tue, 15 Dec 2015 14:42:29 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Luca Abeni <luca.abeni@...tn.it>
Cc:	Vincent Guittot <vincent.guittot@...aro.org>,
	Steve Muckle <steve.muckle@...aro.org>,
	Ingo Molnar <mingo@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Juri Lelli <Juri.Lelli@....com>,
	Patrick Bellasi <patrick.bellasi@....com>,
	Michael Turquette <mturquette@...libre.com>
Subject: Re: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in
 scale_rt_capacity

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.
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