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

Marcel Ziswiler
2025-06-18T11:24:48Z

Hi Juri

On Tue, 2025-06-17 at 14:21 +0200, Juri Lelli wrote:
> On 02/06/25 16:59, Marcel Ziswiler wrote:
> > Hi Juri
> >
> > On Thu, 2025-05-29 at 11:39 +0200, Juri Lelli wrote:
>
> ...
>
> > > It should help us to better understand your setup and possibly reproduce
> > > the problem you are seeing.
>
> OK, it definitely took a while (apologies), but I think I managed to
> reproduce the issue you are seeing.

No need to apologies, I know how hard it can be trying to bring up random stuff in the Linux world : )

> I added SCHED_FLAG_RECLAIM support to rt-app [1], so it's easier for me
> to play with the taskset and got to the following two situations when
> running your coreX taskset on CPU1 of my system (since the issue is
> already reproducible, I think it's OK to ignore the other tasksets as
> they are running isolated on different CPUs anyway).
>
> This is your coreX taskset, in which the last task is the bad behaving one that
> will run without/with RECLAIM in the test.
>
> > sched_deadline = sched_period | sched_runtime | CP max run time 90% of sched_runtime | utilisation |
> > reclaim |
> > -- | -- | -- | -- | -- |
> >  5 ms  | 0.15 ms | 0.135 ms |  3.00% | no |
> > 10 ms  | 1.8 ms  | 1.62 ms  | 18.00% | no |
> > 10 ms  | 2.1 ms  | 1.89 ms  | 21.00% | no |
> > 14 ms  | 2.3 ms  | 2.07 ms  | 16.43% | no |
> > 50 ms  | 8.0 ms  | 7.20 ms  | 16:00% | no |
> > 10 ms  | 0.5 ms  | **1      |  5.00% | no |
>
> Without reclaim everything looks good (apart from the 1st tasks that I
> think suffers a bit from the granularity/precision of rt-app runtime
> loop):
>
> https://github.com/jlelli/misc/blob/main/deadline-no-reclaim.png

Yeah, granularity/precision is definitely a concern. We initially even started off with 1 ms sched_deadline =
sched_period for task 1 but neither of our test systems (amd64-based Intel NUCs and aarch64-based RADXA
ROCK5Bs) was able to handle that very well. So we opted to increase it to 5 ms which is still rather stressful.

> Order is the same as above, last tasks gets constantly throttled and
> makes no harm to the rest.
>
> With reclaim (only last misbehaving task) we indeed seem to have a problem:
>
> https://github.com/jlelli/misc/blob/main/deadline-reclaim.png
>
> Essentially all other tasks are experiencing long wakeup delays that
> cause deadline misses. The bad behaving task seems to be able to almost
> monopolize the CPU. Interesting to notice that, even if I left max
> available bandwidth to 95%, the CPU is busy at 100%.

Yeah, pretty much completely overloaded.

> So, yeah, Luca, I think we have a problem. :-)
>
> Will try to find more time soon and keep looking into this.

Thank you very much and just let me know if I can help in any way.

> Thanks,
> Juri
>
> 1 - https://github.com/jlelli/rt-app/tree/reclaim

BTW: I will be talking at the OSS NA/ELC next week in Denver should any of you folks be around.

Cheers

Marcel