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

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Feb 02, 2026 at 10:13:26PM +0100, Peter Zijlstra wrote:
> 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.

Something like so perhaps?

---
diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 59e650f9d436..884bdf7a292f 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -340,7 +340,7 @@ static ssize_t sched_server_write_common(struct file *filp, const char __user *u
 	long cpu = (long) ((struct seq_file *) filp->private_data)->private;
 	struct rq *rq = cpu_rq(cpu);
 	struct sched_dl_entity *dl_se = (struct sched_dl_entity *)server;
-	u64 runtime, period;
+	u64 old_runtime, runtime, period;
 	int retval = 0;
 	size_t err;
 	u64 value;
@@ -352,7 +352,7 @@ static ssize_t sched_server_write_common(struct file *filp, const char __user *u
 	scoped_guard (rq_lock_irqsave, rq) {
 		bool is_active;
 
-		runtime = dl_se->dl_runtime;
+		old_runtime = runtime = dl_se->dl_runtime;
 		period = dl_se->dl_period;
 
 		switch (param) {
@@ -382,17 +382,20 @@ static ssize_t sched_server_write_common(struct file *filp, const char __user *u
 
 		retval = dl_server_apply_params(dl_se, runtime, period, 0);
 
-		if (!runtime)
-			printk_deferred("%s server disabled in CPU %d, system may crash due to starvation.\n",
-					server == &rq->fair_server ? "Fair" : "Ext", cpu_of(rq));
-
-		if (is_active && runtime)
+		if (runtime)
 			dl_server_start(dl_se);
 
 		if (retval < 0)
 			return retval;
 	}
 
+	if (!!old_runtime ^ !!runtime) {
+		pr_info("%s server %sabled in CPU %d, system may crash due to starvation.\n",
+			server == &rq->fair_server ? "Fair" : "Ext",
+			runtime ? "en" : "dis",
+			cpu_of(rq));
+	}
+
 	*ppos += cnt;
 	return cnt;
 }