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SubjectRe: [RFC v4 0/6] CPU reclaiming for SCHED_DEADLINE
On Wed, 11 Jan 2017 15:06:47 +0000
Juri Lelli <juri.lelli@arm.com> wrote:

> On 11/01/17 13:39, Luca Abeni wrote:
> > Hi Juri,
> > (I reply from my new email address)
> >
> > On Wed, 11 Jan 2017 12:19:51 +0000
> > Juri Lelli <juri.lelli@arm.com> wrote:
> > [...]
> > > > > For example, with my taskset, with a hypothetical perfect
> > > > > balance of the whole runqueue, one possible scenario is:
> > > > >
> > > > > CPU 0 1 2 3
> > > > > # TASKS 3 3 3 2
> > > > >
> > > > > In this case, CPUs 0 1 2 are with 100% of local utilization.
> > > > > Thus, the current task on these CPUs will have their runtime
> > > > > decreased by GRUB. Meanwhile, the luck tasks in the CPU 3
> > > > > would use an additional time that they "globally" do not have
> > > > > - because the system, globally, has a load higher than the
> > > > > 66.6...% of the local runqueue. Actually, part of the time
> > > > > decreased from tasks on [0-2] are being used by the tasks on
> > > > > 3, until the next migration of any task, which will change
> > > > > the luck tasks... but without any guaranty that all tasks
> > > > > will be the luck one on every activation, causing the problem.
> > > > >
> > > > > Does it make sense?
> > > >
> > > > Yes; but my impression is that gEDF will migrate tasks so that
> > > > the distribution of the reclaimed CPU bandwidth is almost
> > > > uniform... Instead, you saw huge differences in the
> > > > utilisations (and I do not think that "compressing" the
> > > > utilisations from 100% to 95% can decrease the utilisation of a
> > > > task from 33% to 25% / 26%... :)
> > >
> > > I tried to replicate Daniel's experiment, but I don't see such a
> > > skewed allocation. They get a reasonably uniform bandwidth and the
> > > trace looks fairly good as well (all processes get to run on the
> > > different processors at some time).
> >
> > With some effort, I replicated the issue noticed by Daniel... I
> > think it also depends on the CPU speed (and on good or bad luck :),
> > but the "unfair" CPU allocation can actually happen.
>
> Yeah, actual allocation in general varies. I guess the question is: do
> we care? We currently don't load balance considering utilizations,
> only dynamic deadlines matter.

Right... But the problem is that with the version of GRUB I proposed
this unfairness can result in some tasks receiving less CPU time than
the guaranteed amount (because some other tasks receive much more). I
think there are at least two possible ways to fix this (without
changing the migration strategy), and I am working on them...
(hopefully, I'll post something in next week)


> > > I was expecting that the task could consume 0.5 worth of bandwidth
> > > with the given global limit. Is the current behaviour intended?
> > >
> > > If we want to change this behaviour maybe something like the
> > > following might work?
> > >
> > > delta_exec = (delta * to_ratio((1ULL << 20) -
> > > rq->dl.non_deadline_bw, rq->dl.running_bw)) >> 20
> > My current patch does
> > (delta * rq->dl.running_bw * rq->dl.deadline_bw_inv) >> 20
> > >> 8; where rq->dl.deadline_bw_inv has been set to
> > to_ratio(global_rt_runtime(), global_rt_period()) >> 12;
> >
> > This seems to work fine, and should introduce less overhead than
> > to_ratio().
> >
>
> Sure, we don't want to do divisions if we can. Why the intermediate
> right shifts, though?

I wrote it like this to remember that ">> 20" comes from how
"to_ratio()" computes the utilization, and the additional ">> 8"
comes from the fact that deadline_bw_inv is shifted left by 8, to avoid
losing precision (I used 8 insted of 20 so that the computation can be
- hopefully - performed on 32 bits... Of course I can revise this if
needed).

If needed I can change the ">> 20 >> 8" in ">> 28", or remove the
">> 12" from the deadline_bw_inv conmputation (so that we can use
">> 40" or ">> 20 >> 20" in grub_reclaim()).


Thanks,
Luca

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