Re: [PATCH v2 3/2] sched/deadline: Check bandwidth overflow earlier for hotplug

[Juri Lelli <juri.lelli@redhat.com>]

On 25/02/25 00:02, Qais Yousef wrote:
> On 02/24/25 10:27, Juri Lelli wrote:
> 
> > > Okay I see. The issue though is that for a DL system with power management
> > > features on that warrant to wake up a sugov thread to update the frequency is
> > > sort of half broken by design. I don't see the benefit over using RT in this
> > > case. But I appreciate I could be misguided. So take it easy on me if it is
> > > obviously wrong understanding :) I know in Android usage of DL has been
> > > difficult, but many systems ship with slow switch hardware.
> > > 
> > > How does DL handle the long softirqs from block and network layers by the way?
> > > This has been in a practice a problem for RT tasks so they should be to DL.
> > > sugov done in stopper should be handled similarly IMHO. I *think* it would be
> > > simpler to masquerade sugov thread as irq pressure.
> > 
> > Kind of a trick question :), as DL doesn't handle this kind of
> 
> :-)
> 
> > load/pressure explicitly. It is essentially agnostic about it. From a
> > system design point of view though, I would say that one should take
> > that into account and maybe convert sensible kthreads to DL, so that the
> > overall bandwidth can be explicitly evaluated. If one doesn't do that
> > probably a less sound approach is to treat anything not explicitly
> > scheduled by DL, but still required from a system perspective, as
> > overload and be more conservative when assigning bandwidth to DL tasks
> > (i.e. reduce the maximum amount of available bandwidth, so that the
> > system doesn't get saturated).
> 
> Maybe I didn't understand your initial answer properly. But what I got is that
> we set as DL to do what you just suggested of converting it kthread to DL to
> take its bandwidth into account. But we have been lying about bandwidth so far
> and it was ignored? (I saw early bailouts of SCHED_FLAG_SUGOV was set in
> bandwidth related operations)

Ignored as to have something 'that works'. :)

But, it's definitely far from being good.

> > > You can use the rate_limit_us as a potential guide for how much bandwidth sugov
> > > needs if moving it to another class really doesn't make sense instead?
> > 
> > Or maybe try to estimate/measure how much utilization sugov threads are
> > effectively using while running some kind of workload of interest and
> > use that as an indication for DL runtime/period.
> 
> I don't want to side track this thread. So maybe I should start a new thread to
> discuss this. You might have seen my other series on consolidating cpufreq
> updates. I'm not sure sugov can have a predictable period. Maybe runtime, but
> it could run repeatedly, or it could be quite for a long time.

Doesn't need to have a predictable period. Sporadic (activations are not
periodic) tasks work well with DEADLINE if one is able to come up with a
sensible bandwidth allocation for them. So for sugov (and other
kthreads) the system designer should be thinking about the amount of CPU
to give to each kthread (runtime/period) and the granularity of such
allocation (period).

> TBH I always though we use DL because it was the highest sched_class that is
> not a stopper.
> 
> Anyway. Happy to take this discussion into another thread if this is better.
> I didn't mean to distract from debugging the reported issue.

No worries! But, a separate thread might help to get more eyes on this,
I agree.

Best,
Juri