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

Qais Yousef
2025-02-16T16:33:44Z

On 02/13/25 07:20, Juri Lelli wrote:
> On 12/02/25 19:22, Dietmar Eggemann wrote:
> > On 11/02/2025 11:42, Juri Lelli wrote:
>
> ...
>
> > > What about we actually ignore them consistently? We already do that for
> > > admission control, so maybe we can do that when rebuilding domains as
> > > well (until we find maybe a better way to deal with them).
> > >
> > > Does the following make any difference?
> >
> > It at least seems to solve the issue. And like you mentioned on irc, we
> > don't know the bw req of sugov anyway.
> >
> > So with this change we start with 'dl_bw->total_bw = 0' even w/ sugov tasks.
> >
> > dl_rq[0]:
> > .dl_nr_running : 0
> > .dl_bw->bw : 996147
> > .dl_bw->total_bw : 0 <-- !
> >
> > IMHO, people who want to run serious DL can always check whether there
> > are already these infrastructural DL tasks or even avoid schedutil.
>
> It definitely not ideal and admittedly gross, but not worse than what we
> are doing already considering we ignore sugovs at AC and the current
> bandwidth allocation its there only to help with PI. So, duck tape. :/
>
> A more proper way to work with this would entail coming up with sensible
> bandwidth allocation for sugovs, but that's most probably hardware
> specific, so I am not sure how we can make that general enough.

I haven't been following the problem closely, but one thing I was considering
and I don't know if it makes sense to you and could help with this problem too.
Shall we lump sugov with stopper class or create a new sched_class (seems
unnecessary, I think stopper should do)? With the consolidate cpufreq update
patch I've been working on Vincent raised issues with potential new ctx switch
and to improve that I needed to look at improving sugov wakeup path. If we
decouple it from DL I think that might fix your problem here and could allow us
to special case it for other problems like the ones I faced more easily without
missing up with DL.

Has the time come to consider retire the simple solution of making sugov a fake
DL task?

>
> Anyway, looks like Jon was still seeing the issue. I asked him to verify
> he is using all the proposed changes. Let's see what he reports.
>
> Best,
> Juri
>