sched/debug: Stop and start server based on if it was active

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.

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Juri Lelli <juri.lelli@redhat.com>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Acked-by: Tejun Heo <tj@kernel.org>
Tested-by: Christian Loehle <christian.loehle@arm.com>
Link: https://patch.msgid.link/20260126100050.3854740-4-arighi@nvidia.com

Author: Joel Fernandes

kernel/sched/debug.c

@@ -348,6 +348,8 @@ 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;
 
@@ -370,16 +372,19 @@ 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);
 
 		if (!runtime)
 			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)