Re: SCHED_DEADLINE tasks missing their deadline with SCHED_FLAG_RECLAIM jobs in the mix (using GRUB)

Juri Lelli
2025-06-20T09:58:23Z

On 20/06/25 11:37, luca abeni wrote:
> Hi Juri,
>
> On Fri, 20 Jun 2025 11:29:52 +0200
> Juri Lelli <juri.lelli@redhat.com> wrote:
> [...]
> > I have been playing a little more with this and noticed (by chance)
> > that after writing a value on sched_rt_runtime_us (even the 950000
> > default) this seem to 'work' - I don't see deadline misses anymore.
> >
> > I thus have moved my attention to GRUB related per-cpu variables [1]
> > and noticed something that looks fishy with extra_bw: after boot and
> > w/o any DEADLINE tasks around (other than dl_servers) all dl_rqs have
> > different values [2]. E.g.,
> >
> > extra_bw : (u64)447170
> > extra_bw : (u64)604454
> [...]
> > So, this might be one thing to look at, but I am honestly still
> > confused by why we have weird numbers as the above after boot. Also a
> > bit confused by the actual meaning and purpose of the 5 GRUB
> > variables we have to deal with.
>
> Sorry about that... I was under the impression they were documented in
> some comments, but I might be wrong...

No worries! I am also culpable, as I did test and review the patches. :)
extra_bw in particular I believe can benefit from a bit of attention.

> > Luca, Vineeth (for the recent introduction of max_bw), maybe we could
> > take a step back and re-check (and maybe and document better :) what
> > each variable is meant to do and how it gets updated?
>
> I am not sure about the funny values initially assigned to these
> variables, but I can surely provide some documentation about what these
> variables represent... I am going to look at this and I'll send some
> comments or patches.

Thanks a lot! I am also continuing to dig.