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

[Andrea Righi <arighi@nvidia.com>]

On Thu, Nov 06, 2025 at 08:13:39AM +0100, Juri Lelli wrote:
> 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.

Should we just call dl_server_remove_params() when runtime is zero?

Thanks,
-Andrea