Re: [PATCH v2 0/4] sched/fair: cpu.max.runtime and idle refill for CPU credits

[Fernand Sieber <sieberf@amazon.com>]

Hi Tejun,

Thank you for your prompt feedback.

On Tue, 10 Nov 2026 10:10:41 -1000, Tejun Heo wrote:
> If a burstable budget is something useful to add to fair, can you do
> that first and separately? It should be able to stand on its own.

That works for me as a starting point. I will resend just patches 1
and 2 as v3.

> Seamlessly switching tasks between the classes in partial mode has a lot
> of other integration problems beyond this one
> [...]
> The sched_ext part can just be a feature of your own BPF scheduler,
> which you seem to be using already. There, the credit level is just
> state in your BPF maps which your userspace can read and set, so the
> maximalist version can wait until the need is proven. If you want a
> handover between your scheduler and fair, that can be done from
> userspace.

The minimal viable bar for us is to never lose track of CPU credits.
With patches 1 and 2, the userspace handover you suggest covers the
planned transitions: on roll out and roll back, userspace reads
cpu.max.runtime before the switch and writes it back after. The one
case it cannot cover is an unplanned exit, a BPF scheduler erroring out
or killed by the watchdog and falling back to fair, where there is no
window for userspace to hand the level back.

Thanks,
Fernand



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