linux-kernel - Re: SCHED_DEADLINE tasks missing their deadline with SCHED_FLAG

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Message-ID: <20250620113745.6833bccb@luca64>
Date: Fri, 20 Jun 2025 11:37:45 +0200
From: luca abeni <luca.abeni@...tannapisa.it>
To: Juri Lelli <juri.lelli@...hat.com>
Cc: Marcel Ziswiler <marcel.ziswiler@...ethink.co.uk>,
 linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>, Peter
 Zijlstra <peterz@...radead.org>, Vineeth Pillai <vineeth@...byteword.org>
Subject: Re: SCHED_DEADLINE tasks missing their deadline with
 SCHED_FLAG_RECLAIM jobs in the mix (using GRUB)

Hi Juri,

On Fri, 20 Jun 2025 11:29:52 +0200
Juri Lelli <juri.lelli@...hat.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...


> 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.


			Luca