linux-kernel - Re: [RFC v5 2/9] sched/deadline: improve the tracking of active utilization

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Date:   Mon, 27 Mar 2017 09:45:21 +0100
From:   Juri Lelli <juri.lelli@....com>
To:     Luca Abeni <luca.abeni@...tannapisa.it>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Claudio Scordino <claudio@...dence.eu.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tommaso Cucinotta <tommaso.cucinotta@...up.it>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>
Subject: Re: [RFC v5 2/9] sched/deadline: improve the tracking of active
 utilization

On 27/03/17 09:43, Luca Abeni wrote:
> Hi Juri,
> 
> On Mon, 27 Mar 2017 08:17:45 +0100
> Juri Lelli <juri.lelli@....com> wrote:
> [...]
> > > > In general I feel it would be nice to have a state diagram
> > > > included somewhere near these two functions. It would be nice to
> > > > not have to dig out the PDF every time.  
> > > 
> > > Ok... Since I am not good at ascii art, would it be ok to add a
> > > textual description? If yes, I'll add a comment like:
> > > "
> > > The utilization of a task is added to the runqueue's active
> > > utilization when the task becomes active (is enqueued in the
> > > runqueue), and is  
> > 
> > Is enqueued for the first time on a new period, maybe? It seems to be
> > contradictory w.r.t. what below (if wakeup before 0 lag time)
> > otherwise.
> I think it should be "is enqueued in the runqueue and was previously
> not active" (I did not write the "and was previously not active" to

Right.

> avoid complicanting the sentence even more... But this
> "simplification" was not a good idea :). The fact that this happens in a
> new period or not is (in my understanding) irrelevant...
> 
> 
> > > removed when the task becomes inactive. A task does not become
> > > immediately inactive when it blocks, but becomes inactive at the so
> > > called "0 lag time"; so, we setup the "inactive timer" to fire at
> > > the "0 lag time". When the "inactive timer" fires, the task
> > > utilization is removed from the runqueue's active utilization. If
> > > the task wakes up again on the same runqueue before the "0 lag
> > > time", the active utilization must not be changed and the "inactive
> > > timer" must be cancelled. If the task wakes up again on a different
> > > runqueue before the "0 lag time", then the task's utilization must
> > > be removed from the previous runqueue's active utilization and must
> > > be added to the new runqueue's active utilization.
> > > In order to avoid races between a task waking up on a runqueue
> > > while the "inactive timer" is running on a different CPU, the
> > > "dl_non_contending" flag is used to indicate that a task is not on
> > > a runqueue but is active (so, the flag is set when the task blocks
> > > and is cleared when the "inactive timer" fires or when the task
> > > wakes up). "
> > > (if this is ok, where can I add this comment?)
> > >   
> > 
> > Thanks for this Luca. Not sure it adds much to your text above, but we
> > might want to consider adding something like below?
> > 
> > --->8---  
> >            1st enqueue       +------------------+
> >                              |                  |
> >            +---------------->+ ACTIVEcontending |
> >            |                 |                  |
> >            |                 +----+------+------+
> >            |                      |      ^
> >            |                      |      |
> >   +--------+-------+              |      |
> >   |                |     dequeue  |      |  wakeup before
> >   |    INACTIVE    |              |      |  0 lag time
> >   |                |              |      |
> >   +--------+-------+              |      |
> >            ^                      |      |
> >            |                      V      |
> >            |                 +----+------+------+
> >            |                 |                  |
> >            +-----------------+ ACTIVEnonCONTEND |
> >                              |                  |
> >             0 lag time       +------------------+
> >             elapsed
> > --->8---  
> 
> I am not sure if introducing the "active non contending" name is a good
> idea or not (see my previous email), but I am not the best person to
> decide this... If people like this figure, I am more than happy to add
> it :)
> (but then maybe we can change "0 lag time elapsed" with "inactive timer
> fires" and we can display in the figure the state of the
> "dl_non_contending"/"inactive_timer_armed" flag)
> 

Sure. Let's see what people think about what you say in the other email
and I'll update the figure accordingly.