Chris reported that commit 5f6bd380c7bd ("sched/rt: Remove default
bandwidth control") caused a significant dip in his favourite
benchmark of the day. Simply disabling dl_server cured things.

His workload hammers the 0->1, 1->0 transitions, and the
dl_server_{start,stop}() overhead kills it -- fairly obviously a bad
idea in hind sight and all that.

Change things around to only disable the dl_server when there has not
been a fair task around for a whole period. Since the default period
is 1 second, this ensures the benchmark never trips this, overhead
gone.

Fixes: 557a6bfc662c ("sched/fair: Add trivial fair server")
Reported-by: Chris Mason <clm@meta.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 include/linux/sched.h   |    1 +
 kernel/sched/deadline.c |   31 +++++++++++++++++++++++++++----
 2 files changed, 28 insertions(+), 4 deletions(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -702,6 +702,7 @@ struct sched_dl_entity {
 	unsigned int			dl_defer	  : 1;
 	unsigned int			dl_defer_armed	  : 1;
 	unsigned int			dl_defer_running  : 1;
+	unsigned int			dl_server_idle    : 1;
 
 	/*
 	 * Bandwidth enforcement timer. Each -deadline task has its
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1215,6 +1215,8 @@ static void __push_dl_task(struct rq *rq
 /* a defer timer will not be reset if the runtime consumed was < dl_server_min_res */
 static const u64 dl_server_min_res = 1 * NSEC_PER_MSEC;
 
+static bool dl_server_stopped(struct sched_dl_entity *dl_se);
+
 static enum hrtimer_restart dl_server_timer(struct hrtimer *timer, struct sched_dl_entity *dl_se)
 {
 	struct rq *rq = rq_of_dl_se(dl_se);
@@ -1234,6 +1236,7 @@ static enum hrtimer_restart dl_server_ti
 
 		if (!dl_se->server_has_tasks(dl_se)) {
 			replenish_dl_entity(dl_se);
+			dl_server_stopped(dl_se);
 			return HRTIMER_NORESTART;
 		}
 
@@ -1639,8 +1642,10 @@ void dl_server_update_idle_time(struct r
 void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec)
 {
 	/* 0 runtime = fair server disabled */
-	if (dl_se->dl_runtime)
+	if (dl_se->dl_runtime) {
+		dl_se->dl_server_idle = 0;
 		update_curr_dl_se(dl_se->rq, dl_se, delta_exec);
+	}
 }
 
 void dl_server_start(struct sched_dl_entity *dl_se)
@@ -1663,7 +1668,7 @@ void dl_server_start(struct sched_dl_ent
 		setup_new_dl_entity(dl_se);
 	}
 
-	if (!dl_se->dl_runtime)
+	if (!dl_se->dl_runtime || dl_se->dl_server_active)
 		return;
 
 	dl_se->dl_server_active = 1;
@@ -1672,7 +1677,7 @@ void dl_server_start(struct sched_dl_ent
 		resched_curr(dl_se->rq);
 }
 
-void dl_server_stop(struct sched_dl_entity *dl_se)
+static void __dl_server_stop(struct sched_dl_entity *dl_se)
 {
 	if (!dl_se->dl_runtime)
 		return;
@@ -1684,6 +1689,24 @@ void dl_server_stop(struct sched_dl_enti
 	dl_se->dl_server_active = 0;
 }
 
+static bool dl_server_stopped(struct sched_dl_entity *dl_se)
+{
+	if (!dl_se->dl_server_active)
+		return false;
+
+	if (dl_se->dl_server_idle) {
+		__dl_server_stop(dl_se);
+		return true;
+	}
+
+	dl_se->dl_server_idle = 1;
+	return false;
+}
+
+void dl_server_stop(struct sched_dl_entity *dl_se)
+{
+}
+
 void dl_server_init(struct sched_dl_entity *dl_se, struct rq *rq,
 		    dl_server_has_tasks_f has_tasks,
 		    dl_server_pick_f pick_task)
@@ -2435,7 +2458,7 @@ static struct task_struct *__pick_task_d
 	if (dl_server(dl_se)) {
 		p = dl_se->server_pick_task(dl_se);
 		if (!p) {
-			if (dl_server_active(dl_se)) {
+			if (!dl_server_stopped(dl_se)) {
 				dl_se->dl_yielded = 1;
 				update_curr_dl_se(rq, dl_se, 0);
 			}

Hi,

On 20/05/25 11:45, Peter Zijlstra wrote:
> Chris reported that commit 5f6bd380c7bd ("sched/rt: Remove default
> bandwidth control") caused a significant dip in his favourite
> benchmark of the day. Simply disabling dl_server cured things.
> 
> His workload hammers the 0->1, 1->0 transitions, and the
> dl_server_{start,stop}() overhead kills it -- fairly obviously a bad
> idea in hind sight and all that.
> 
> Change things around to only disable the dl_server when there has not
> been a fair task around for a whole period. Since the default period
> is 1 second, this ensures the benchmark never trips this, overhead
> gone.
> 
> Fixes: 557a6bfc662c ("sched/fair: Add trivial fair server")
> Reported-by: Chris Mason <clm@meta.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  include/linux/sched.h   |    1 +
>  kernel/sched/deadline.c |   31 +++++++++++++++++++++++++++----
>  2 files changed, 28 insertions(+), 4 deletions(-)
> 
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -702,6 +702,7 @@ struct sched_dl_entity {
>  	unsigned int			dl_defer	  : 1;
>  	unsigned int			dl_defer_armed	  : 1;
>  	unsigned int			dl_defer_running  : 1;
> +	unsigned int			dl_server_idle    : 1;
>  
>  	/*
>  	 * Bandwidth enforcement timer. Each -deadline task has its
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1215,6 +1215,8 @@ static void __push_dl_task(struct rq *rq
>  /* a defer timer will not be reset if the runtime consumed was < dl_server_min_res */
>  static const u64 dl_server_min_res = 1 * NSEC_PER_MSEC;
>  
> +static bool dl_server_stopped(struct sched_dl_entity *dl_se);
> +
>  static enum hrtimer_restart dl_server_timer(struct hrtimer *timer, struct sched_dl_entity *dl_se)
>  {
>  	struct rq *rq = rq_of_dl_se(dl_se);
> @@ -1234,6 +1236,7 @@ static enum hrtimer_restart dl_server_ti
>  
>  		if (!dl_se->server_has_tasks(dl_se)) {
>  			replenish_dl_entity(dl_se);
> +			dl_server_stopped(dl_se);
>  			return HRTIMER_NORESTART;
>  		}
>  
> @@ -1639,8 +1642,10 @@ void dl_server_update_idle_time(struct r
>  void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec)
>  {
>  	/* 0 runtime = fair server disabled */
> -	if (dl_se->dl_runtime)
> +	if (dl_se->dl_runtime) {
> +		dl_se->dl_server_idle = 0;
>  		update_curr_dl_se(dl_se->rq, dl_se, delta_exec);
> +	}
>  }
>  
>  void dl_server_start(struct sched_dl_entity *dl_se)
> @@ -1663,7 +1668,7 @@ void dl_server_start(struct sched_dl_ent
>  		setup_new_dl_entity(dl_se);
>  	}
>  
> -	if (!dl_se->dl_runtime)
> +	if (!dl_se->dl_runtime || dl_se->dl_server_active)
>  		return;
>  
>  	dl_se->dl_server_active = 1;
> @@ -1672,7 +1677,7 @@ void dl_server_start(struct sched_dl_ent
>  		resched_curr(dl_se->rq);
>  }
>  
> -void dl_server_stop(struct sched_dl_entity *dl_se)
> +static void __dl_server_stop(struct sched_dl_entity *dl_se)
>  {
>  	if (!dl_se->dl_runtime)
>  		return;
> @@ -1684,6 +1689,24 @@ void dl_server_stop(struct sched_dl_enti
>  	dl_se->dl_server_active = 0;
>  }
>  
> +static bool dl_server_stopped(struct sched_dl_entity *dl_se)
> +{
> +	if (!dl_se->dl_server_active)
> +		return false;
> +
> +	if (dl_se->dl_server_idle) {
> +		__dl_server_stop(dl_se);
> +		return true;
> +	}
> +
> +	dl_se->dl_server_idle = 1;
> +	return false;
> +}
> +
> +void dl_server_stop(struct sched_dl_entity *dl_se)
> +{
> +}

What if we explicitly set the server to idle (instead of ignoring the
stop) where this gets called in dequeue_entities()? Also, don't we need
to actually stop the server if we are changing its parameters from
sched_fair_server_write()?

Thanks,
Juri

On Tue, Jun 03, 2025 at 06:03:12PM +0200, Juri Lelli wrote:
> Hi,
> 

> > @@ -1684,6 +1689,24 @@ void dl_server_stop(struct sched_dl_enti
> >  	dl_se->dl_server_active = 0;
> >  }
> >  
> > +static bool dl_server_stopped(struct sched_dl_entity *dl_se)
> > +{
> > +	if (!dl_se->dl_server_active)
> > +		return false;
> > +
> > +	if (dl_se->dl_server_idle) {
> > +		__dl_server_stop(dl_se);
> > +		return true;
> > +	}
> > +
> > +	dl_se->dl_server_idle = 1;
> > +	return false;
> > +}
> > +
> > +void dl_server_stop(struct sched_dl_entity *dl_se)
> > +{
> > +}
> 
> What if we explicitly set the server to idle (instead of ignoring the
> stop) where this gets called in dequeue_entities()?

That would break thing; we want to detect if it was ever !idle in the
period.

> Also, don't we need to actually stop the server if we are changing its
> parameters from sched_fair_server_write()?

Quite - let me just remove the offending callsites them.

Would this explain this massive regression 0day reported here? Seems
weird.

Anyway, let me go update the patch.