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

[Andrea Righi <arighi@nvidia.com>]

On Tue, Feb 03, 2026 at 11:34:07AM +0100, Peter Zijlstra wrote:
> On Mon, Feb 02, 2026 at 11:37:31PM +0100, Andrea Righi wrote:
> 
> > Or:
> > 
> >     pr_info("%s server %sabled in CPU %d%s\n",
> >               server == &rq->fair_server ? "Fair" : "Ext",
> >               runtime ? "en" : "dis",
> >               cpu_of(rq),
> >               runtime ? "" : ", system may crash due to starvation");
> 
> Yeah, I noticed it was a bit wonkey. I made it thus.
> 
> > > +	}
> > > +
> > >  	*ppos += cnt;
> > >  	return cnt;
> > >  }
> > 
> > I like that, it should fix the issue.
> 
> There is one more issue when dl_server_apply_params() fails, in that
> case we should test old_runtime to determine if we should (re)start the
> dl_server.
> 
> I've ended up with this.

LGTM, I also re-ran all my stress tests with this applied, everything is
working great and the runtime=0 issue is fixed.

Tested-by: Andrea Righi <arighi@nvidia.com>

Thanks!
-Andrea

> Subject: sched/debug: Fix dl_server (re)start conditions
> From: Peter Zijlstra <peterz@infradead.org>
> Date: Tue Feb 3 11:05:12 CET 2026
> 
> There are two problems with sched_server_write_common() that can cause the
> dl_server to malfunction upon attempting to change the parameters:
> 
> 1) when, after having disabled the dl_server by setting runtime=0, it is
>    enabled again while tasks are already enqueued. In this case is_active would
>    still be 0 and dl_server_start() would not be called.
> 
> 2) when dl_server_apply_params() would fail, runtime is not applied and does
>    not reflect the new state.
> 
> Instead have dl_server_start() check its actual dl_runtime, and have
> sched_server_write_common() unconditionally (re)start the dl_server. It will
> automatically stop if there isn't anything to do, so spurious activation is
> harmless -- while failing to start it is a problem.
> 
> While there, move the printk out of the locked region and make it symmetric,
> also printing on enable.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  kernel/sched/deadline.c |    5 ++---
>  kernel/sched/debug.c    |   32 ++++++++++++++------------------
>  2 files changed, 16 insertions(+), 21 deletions(-)
> 
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1784,7 +1784,7 @@ void dl_server_start(struct sched_dl_ent
>  {
>  	struct rq *rq = dl_se->rq;
>  
> -	if (!dl_server(dl_se) || dl_se->dl_server_active)
> +	if (!dl_server(dl_se) || dl_se->dl_server_active || !dl_se->dl_runtime)
>  		return;
>  
>  	/*
> @@ -1882,7 +1882,6 @@ int dl_server_apply_params(struct sched_
>  	int cpu = cpu_of(rq);
>  	struct dl_bw *dl_b;
>  	unsigned long cap;
> -	int retval = 0;
>  	int cpus;
>  
>  	dl_b = dl_bw_of(cpu);
> @@ -1914,7 +1913,7 @@ int dl_server_apply_params(struct sched_
>  	dl_se->dl_bw = to_ratio(dl_se->dl_period, dl_se->dl_runtime);
>  	dl_se->dl_density = to_ratio(dl_se->dl_deadline, dl_se->dl_runtime);
>  
> -	return retval;
> +	return 0;
>  }
>  
>  /*
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -338,9 +338,9 @@ static ssize_t sched_server_write_common
>  					 void *server)
>  {
>  	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;
> +	struct rq *rq = cpu_rq(cpu);
>  	int retval = 0;
>  	size_t err;
>  	u64 value;
> @@ -350,9 +350,7 @@ static ssize_t sched_server_write_common
>  		return err;
>  
>  	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) {
> @@ -374,25 +372,23 @@ static ssize_t sched_server_write_common
>  			return  -EINVAL;
>  		}
>  
> -		is_active = dl_server_active(dl_se);
> -		if (is_active) {
> -			update_rq_clock(rq);
> -			dl_server_stop(dl_se);
> -		}
> -
> +		update_rq_clock(rq);
> +		dl_server_stop(dl_se);
>  		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)
> -			dl_server_start(dl_se);
> +		dl_server_start(dl_se);
>  
>  		if (retval < 0)
>  			return retval;
>  	}
>  
> +	if (!!old_runtime ^ !!runtime) {
> +		pr_info("%s server %sabled on CPU %d%s.\n",
> +			server == &rq->fair_server ? "Fair" : "Ext",
> +			runtime ? "en" : "dis",
> +			cpu_of(rq),
> +			runtime ? "" : ", system may malfunction due to starvation");
> +	}
> +
>  	*ppos += cnt;
>  	return cnt;
>  }