Re: [REGRESSION 6.12 -> 6.18] fair (dl-)server delivers no bandwidth to CFS under a CPU-bound RT task on a nohz_full isolated core

[Juri Lelli]

Hello,

Thanks for reporting!

On 09/07/26 19:59, Ionut Nechita (Wind River) wrote:
> Hi,
> 
> I'm chasing what looks like a fair (dl-)server regression between 6.12 and
> 6.18 and wanted to ask whether this behaviour is known/expected before I
> spend build cycles on a full bisect.
> 
> Question in one line: on a nohz_full isolated core, with a CPU-bound
> SCHED_RR task pinned to it, should the fair_server still guarantee its
> 50ms/1s (5%) to a co-scheduled SCHED_OTHER task? On 6.12 it does; on 6.18
> it delivers essentially nothing (~0.2%).

I believe it should still guarantee the 5%.

Can you please check if the following fixes the issue?

https://lore.kernel.org/lkml/20260513-upstream-fix-dlserver-nohzfull-b4-v2-1-d3e9cbe5c845@xxxxxxxxxx/

Thanks,
Juri