Re: [PATCH 3/7] sched/debug: Stop and start server based on if it was active

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jan 26, 2026 at 10:59:01AM +0100, 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.
> 
> Tested-by: Christian Loehle <christian.loehle@arm.com>
> 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 93f009e1076d8..dd793f8f3858a 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 && runtime)
>  			dl_server_start(&rq->fair_server);
>  
>  		if (retval < 0)

Suppose runtime was 0, and gets incremented while there are already
tasks enqueued, then the above isn't going to DTRT.