Re: [PATCHSET v3 sched_ext/for-7.2] sched_ext: Auto-manage ext/fair dl_server bandwidth

[Juri Lelli]

Hi Andrea,

On 26/05/26 18:42, Andrea Righi wrote:
> Currently, a fixed bandwidth is reserved at boot for both the fair and ext
> deadline servers, and this reservation remains unchanged unless explicitly
> modified via debugfs. As a result, both servers permanently contribute to global
> bandwidth accounting, regardless of whether a BPF scheduler is active.
> 
> While unused bandwidth can still be reclaimed at runtime by other classes, this
> static reservation prevents RT from fully utilizing available headroom in
> situations where one of the sched_ext or fair class is guaranteed to be inactive
> (for example, when no BPF scheduler is loaded, or when sched_ext runs in full
> mode and replaces fair).
> 
> As discussed at the VIII OSPM summit in Cambridge [1], a better solution would
> be to dynamically register and unregister deadline server bandwidth based on the
> active sched_ext state. This allows the kernel to automatically enable bandwidth
> accounting only for the scheduling class that is currently active, while
> disabling it for inactive ones.
> 
> This patch series implements this automatic register/unregister logic. Moreover,
> the sched_ext total_bw kselftest is also modified to validate the correct
> behavior across the different scheduling configurations and ensure that
> bandwidth accounting follows the expected state transitions.
> 
> [1] https://retis.santannapisa.it/ospm-summit/
> 
> Git tree: git://git.kernel.org/pub/scm/linux/kernel/git/arighi/linux.git dl-server-bw-v3
> 
> Changes in v3:
>  - Don't bypass __dl_overflow() for detached servers in dl_server_apply_params()
>    to reject oversized configs up front (reported by Sashiko)
>  - A potential divide-by-zero in dl_server_apply_params() reported by Sashiko
>    has been fixed in a separate patch (not introduced by this patch set):
>    https://lore.kernel.org/all/20260526100502.575774-1-arighi@xxxxxxxxxx/
>  - Link to v2: https://lore.kernel.org/all/20260526082954.550958-1-arighi@xxxxxxxxxx/

This looks now good to me.

Acked-by: Juri Lelli <juri.lelli@xxxxxxxxxx>

Thanks!
Juri