Re: [PATCH] sched/deadline: Fix replenishment logic for non-deferred servers

[Juri Lelli]

Hi Yuri,

On 20/04/26 18:34, Yuri Andriaccio wrote:
> A special flag of the deadline entities, the 'dl_defer' flag, is explicitly
> documents changing the behaviour of deadline tasks and servers to the special
> deferred behaviour that replenishes and unthrottles the linked dl entity just in
> time so it does not miss its deadline.

Maybe rephrase to make it more direct, like "The dl_defer flag is used
..."?

> Currently, this behaviour is only used by fair-servers (and ext-servers), while
> regular tasks use the original CBS (Constant Bandwidth Server) rules.
> 
> The bug (and fix) which follows is of interest to those deadline server entities
> which do not request the deferred behaviour (i.e., dl_defer = 0):
> whenever a non-deferred deadline entity expends all of its runtime it must start
> the replenishment timer or, if the wake-up instant is too close to the current
> time, the entity must be simply re-enqueued and its runtime replenished.
> Currently, the 'dl_defer' flag is ignored when replenishing any deadline server,
> defaulting to the deferred behaviour, while this fix correctly addresses the
> issue.

I think we might want to make the above more imperative (don't use "this
fix", etc). Simply state what the fix does.

> Unfortunately, no testing of this bugfix is possible as there are currently no
> deadline servers which use the standard non-deferred behaviour. This bug was
> discovered during the development of the Hierarchical CBS patch
> (https://lore.kernel.org/all/20251201124205.11169-1-yurand2000@xxxxxxxxx/) which
> makes extensive use of the non-deferred servers.
> 
> Thanks for your support and have a nice day,
> Yuri

The above (from "Unfortunately..) doesn't belong to the changelog, you
can put it under the --- region in your next version if you want to add
comments of this type.

> Signed-off-by: Yuri Andriaccio <yurand2000@xxxxxxxxx>
> ---

Apart from the suggestions above, the fix makes sense to me (and I trust
it works with the rest of HCBS patches you are working on).

Thanks!
Juri