Re: [PATCH] sched/deadline: Remove redundant code replenishing runtime

From: Byungchul Park
Date: Mon Feb 13 2017 - 18:43:18 EST

On Mon, Feb 13, 2017 at 03:24:55PM +0000, Juri Lelli wrote:
> > > > I think we actually want to replenish and set the next deadline at this
> > > > point of time, not the one that we get when the task will eventually wake up.
> > >
> > > Hello juri,
> > >
> > > But I wonder if it's meaningful to set a next deadline for a 'sleeping
> > > task', which, rather, could be worse because its bandwidth might be
> > > distorted at the time it's woken up.
> > >
> What you mean by 'distorted'. AFAIU, we just want to replenish when
> needed. The instant of time when the task will eventually wake up it is
> something we cannot rely upon, and could introduce errors.
> IIUC, your situation looks like the below


> oooo|-------------------vxxx^ooo
> | | |
> | | |
> sleep/throttle | |
> r. timer |
> wakeup

Sorry for bothering you..

> The task gets throttled while going to sleep, when the replenishment
> timer fires you are proposing we do nothing and we actually replenishing
> using the wakeup rq_clock() as reference. My worry is that, by doing so,
> we make the task potentially loose some of its bandwidth, as we will
> have lost some time (the 3 x-es in the diagram above) when calculating
> its next dynamic deadline.

I meant, when we decide whether it's overflowed in dl_entiry_overflow(),
'right' might be smaller than 'left' because 't' is the time the 3 x-es
already passed.

Of course, here I assumed that runtime ~= 0 and deadline ~= rq_clock
when it was throttled, if scheduler works nicely.

> > > IMHO, it's neat to set its deadline and runtime when being woken up, in
> > > the case already passed its deadline. Am I wrong?
> >
> > And I found that dl_entity_overflow() returns true and replenishes the
> > task unconditionally in update_dl_entity() again when the task is woken
> > up, because 'runtime / (deadline - t) > dl_runtime / dl_period' is true.
> >
> Why 'unconditionally'? It will postpone and replenish if the task is

Not exactly 'unconditially' if my assumption is broken. Sorry for
choosing a word that is not careful.

> going to overflow, if not, it will keep its runtime and deadline we set

I meant the task will be almost always considered 'overflow', as I
explained above. So it will be overwritten again when waking up the task
than keep what we set in timer callback.

> when the replenishment timer fired.