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

From: Juri Lelli
Date: Mon Mar 27 2017 - 04:46:00 EST


On 27/03/17 09:43, Luca Abeni wrote:
> Hi Juri,
>
> On Mon, 27 Mar 2017 08:17:45 +0100
> Juri Lelli <juri.lelli@xxxxxxx> 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.