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

[Peter Zijlstra]

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@xxxxxxxxxx>
> > 
> > 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@xxxxxxx>
> > Acked-by: Tejun Heo <tj@xxxxxxxxxx>
> > Reviewed-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
> > Reviewed-by: Andrea Righi <arighi@xxxxxxxxxx>
> > Signed-off-by: Joel Fernandes <joelagnelf@xxxxxxxxxx>
> > ---
> >  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.

Perhaps simply make that:

	if (runtime)
		dl_server_start();

That might spuriously start the thing, but that should be harmless. It
will just go back to sleep for not finding any tasks to run and all
that.