Re: [PATCH 04/11] sched/deadline: Add support to initialize and remove dl_server bandwidth

[Juri Lelli <juri.lelli@redhat.com>]

Hi,

On 29/10/25 20:08, Andrea Righi wrote:
> During switching from sched_ext to fair tasks and vice-versa, we need
> support for intializing and removing the bandwidth contribution of
> either DL server.

My first and more general/design question is do we strictly need this
automagic bandwidth management. We seem to agree [1] that we want to
move towards explicit dl-server(s) and tasks bandwidth handling, so we
might want to consider leaving the burden completely to whomever might
be configuring the system.

> Add support for handling these transitions.

Anyway, if we still want to do this :) ...

> Moreover, remove references specific to the fair server, in preparation
> for adding the ext server.
> 
> v2: - wait for inactive_task_timer to fire before removing the bandwidth
>       reservation (Juri Lelli)
>     - add WARN_ON_ONCE(!cpus) sanity check in dl_server_apply_params()
>       (Andrea Righi)
> 
> Co-developed-by: Joel Fernandes <joelagnelf@nvidia.com>
> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> ---

...

> +/**
> + * dl_server_remove_params - Remove bandwidth reservation for a DL server
> + * @dl_se: The DL server entity to remove bandwidth for
> + *
> + * This function removes the bandwidth reservation for a DL server entity,
> + * cleaning up all bandwidth accounting and server state.
> + *
> + * Returns: 0 on success, negative error code on failure
> + */
> +int dl_server_remove_params(struct sched_dl_entity *dl_se,
> +			    struct rq *rq, struct rq_flags *rf)
> +{
> +	if (!dl_se->dl_server)
> +		return 0; /* Already disabled */
> +
> +	/*
> +	 * First dequeue if still queued. It should not be queued since
> +	 * we call this only after the last dl_server_stop().
> +	 */
> +	if (WARN_ON_ONCE(on_dl_rq(dl_se)))
> +		dequeue_dl_entity(dl_se, DEQUEUE_SLEEP);
> +
> +	if (hrtimer_try_to_cancel(&dl_se->inactive_timer) == -1) {
> +		rq_unlock_irqrestore(rq, rf);

This seems racy. I fear the moment we release the rq lock something can
slip in and the server(s) state might change?

> +
> +		hrtimer_cancel(&dl_se->inactive_timer);

I am not sure we actually need to force cancel the timer (but still
contradicting myself every time I go back at staring at code :). The way
I believe this should work 'in theory' is

 - we remove a server (either automagic or user sets runtime to 0 -
   which is probably to fix/look at in current implementation as well
   btw)
 - current bandwidth is retained and only freed (and server reset) at
   0-lag (when inactive_timer fires)
 - if server is activated back before 0-lag it will use it's current
   parameters
 - after 0-lag it's a new instance with new parameters

In inactive_timer() we have this behavior for simple tasks, but we skip
__dl_sub() etc for servers (since we clear it up immediately).

In all this I essentially fear that if we clear parameters immediately
one could be able to trick the system by quickly disabling/enabling a
dl-server to let fair/scx tasks execute more than what requested (as
each new enable will be seen as a new instance). But, again, I wasn't
yet able to demonstrate this and I am still uncomfortably uncertain.
Please Peter and others keep me honest.

Also, server parameters changes are root only, so maybe not a big deal?
For scx automagic as well?

Thanks!
Juri

1 - https://lore.kernel.org/lkml/aQiE1ULtInJS6X4R@jlelli-thinkpadt14gen4.remote.csb/