Re: [PATCH 02/11] sched/debug: Stop and start server based on if it was active

Juri Lelli
2025-11-06T07:13:47Z

Hi,

On 29/10/25 20:08, Andrea Righi wrote:
> From: Joel Fernandes <joelagnelf@nvidia.com>
>
> Currently the DL server interface for applying parameters checks
> CFS-internals to identify if the server is active. This is error-prone
> and makes it difficult when adding new servers in the future.
>
> Fix it, by using dl_server_active() which is also used by the DL server
> code to determine if the DL server was started.
>
> Acked-by: Tejun Heo <tj@kernel.org>
> Reviewed-by: Juri Lelli <juri.lelli@redhat.com>
> Reviewed-by: Andrea Righi <arighi@nvidia.com>
> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> ---
> kernel/sched/debug.c | 11 ++++++++---
> 1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> index 6cf9be6eea49a..e71f6618c1a6a 100644
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -354,6 +354,8 @@ static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubu
> return err;
>
> scoped_guard (rq_lock_irqsave, rq) {
> + bool is_active;
> +
> runtime = rq->fair_server.dl_runtime;
> period = rq->fair_server.dl_period;
>
> @@ -376,8 +378,11 @@ static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubu
> return -EINVAL;
> }
>
> - update_rq_clock(rq);
> - dl_server_stop(&rq->fair_server);
> + is_active = dl_server_active(&rq->fair_server);
> + if (is_active) {
> + update_rq_clock(rq);
> + dl_server_stop(&rq->fair_server);
> + }
>
> retval = dl_server_apply_params(&rq->fair_server, runtime, period, 0);
>
> @@ -385,7 +390,7 @@ static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubu
> printk_deferred("Fair server disabled in CPU %d, system may crash due to starvation.\n",
> cpu_of(rq));
>
> - if (rq->cfs.h_nr_queued)
> + if (is_active)
> dl_server_start(&rq->fair_server);

Something that I noticed while reviewing this series is that we still
start back a server even if the user put its runtime to zero (disabling
it) and I don't think we want to do that. It's not of course related to
this change or this series per-se, but something we probably want to fix
independently.

Thanks,
Juri