With the consolidation of put_prev_task/set_next_task(), see
commit 436f3eed5c69 ("sched: Combine the last put_prev_task() and the
first set_next_task()"), we are now skipping the transition between
these two functions when the previous and the next tasks are the same.

As a result, ops.update_idle() is now called only once when the CPU
transitions to the idle class. If the CPU stays active (e.g., through a
call to scx_bpf_kick_cpu()), ops.update_idle() will not be triggered
again since the task remains unchanged (rq->idle).

While this behavior seems generally correct, it can cause issues in
certain sched_ext scenarios.

For example, a BPF scheduler might use logic like the following to keep
the CPU active under specific conditions:

void BPF_STRUCT_OPS(sched_update_idle, s32 cpu, bool idle)
{
	if (!idle)
		return;
	if (condition)
		scx_bpf_kick_cpu(cpu, 0);
}

A call to scx_bpf_kick_cpu() wakes up the CPU, so in theory,
ops.update_idle() should be triggered again until the condition becomes
false. However, this doesn't happen, and scx_bpf_kick_cpu() doesn't
produce the expected effect.

In practice, this change badly impacts performance in user-space
schedulers that rely on ops.update_idle() to activate user-space
components.

For instance, in the case of scx_rustland, performance drops
significantly (e.g., gaming benchmarks fall from ~60fps to ~10fps).

To address this, trigger ops.update_idle() from pick_task_idle() rather
than set_next_task_idle(). This restores the correct behavior of
ops.update_idle() and it allows to fix the performance regression in
scx_rustland.

Fixes: 7c65ae81ea86 ("sched_ext: Don't call put_prev_task_scx() before picking the next task")
Signed-off-by: Andrea Righi <andrea.righi@linux.dev>
---
 kernel/sched/idle.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

ChangeLog v1 -> v2:
  - move the logic from put_prev_set_next_task() to scx_update_idle()

diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index d2f096bb274c..5a10cbc7e9df 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -459,13 +459,13 @@ static void put_prev_task_idle(struct rq *rq, struct task_struct *prev, struct t
 static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool first)
 {
 	update_idle_core(rq);
-	scx_update_idle(rq, true);
 	schedstat_inc(rq->sched_goidle);
 	next->se.exec_start = rq_clock_task(rq);
 }
 
 struct task_struct *pick_task_idle(struct rq *rq)
 {
+	scx_update_idle(rq, true);
 	return rq->idle;
 }
 
-- 
2.47.0

On Tue, Oct 15, 2024 at 12:06:03AM +0200, Andrea Righi wrote:

> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index d2f096bb274c..5a10cbc7e9df 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -459,13 +459,13 @@ static void put_prev_task_idle(struct rq *rq, struct task_struct *prev, struct t
>  static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool first)
>  {
>  	update_idle_core(rq);
> -	scx_update_idle(rq, true);
>  	schedstat_inc(rq->sched_goidle);
>  	next->se.exec_start = rq_clock_task(rq);
>  }
>  
>  struct task_struct *pick_task_idle(struct rq *rq)
>  {
> +	scx_update_idle(rq, true);
>  	return rq->idle;
>  }

Does this do the right thing in the case of core-scheduling doing
pick_task() for force-idle on a remote cpu?

The core-sched case is somewhat special in that the pick can be ignored
-- in which case you're doing a spurious scx_update_idle() call.

On Tue, Oct 15, 2024 at 09:45:26AM +0200, Peter Zijlstra wrote:
> On Tue, Oct 15, 2024 at 12:06:03AM +0200, Andrea Righi wrote:
> 
> > diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> > index d2f096bb274c..5a10cbc7e9df 100644
> > --- a/kernel/sched/idle.c
> > +++ b/kernel/sched/idle.c
> > @@ -459,13 +459,13 @@ static void put_prev_task_idle(struct rq *rq, struct task_struct *prev, struct t
> >  static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool first)
> >  {
> >  	update_idle_core(rq);
> > -	scx_update_idle(rq, true);
> >  	schedstat_inc(rq->sched_goidle);
> >  	next->se.exec_start = rq_clock_task(rq);
> >  }
> >  
> >  struct task_struct *pick_task_idle(struct rq *rq)
> >  {
> > +	scx_update_idle(rq, true);
> >  	return rq->idle;
> >  }
> 
> Does this do the right thing in the case of core-scheduling doing
> pick_task() for force-idle on a remote cpu?
> 
> The core-sched case is somewhat special in that the pick can be ignored
> -- in which case you're doing a spurious scx_update_idle() call.

Hm... that's right. So, what about keeping scx_update_idle() in
set_next_task_idle() and also call it from pick_task(), but only when
rq->curr == rq->idle?

In this way, we should still be able to handle the scx_bpf_kick_cpu()
call from ops.update_idle() properly and, while we might still encounter
spurious calls in the core scheduling case, the idle state provided by
ops.update_idle() will always be correct. So, scx schedulers that want
to implement their own cpu idle state can rely on ops.update_idle().

-Andrea

Hello,

On Tue, Oct 15, 2024 at 12:06:03AM +0200, Andrea Righi wrote:
> @@ -459,13 +459,13 @@ static void put_prev_task_idle(struct rq *rq, struct task_struct *prev, struct t
>  static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool first)
>  {
>  	update_idle_core(rq);
> -	scx_update_idle(rq, true);
>  	schedstat_inc(rq->sched_goidle);
>  	next->se.exec_start = rq_clock_task(rq);
>  }
>  
>  struct task_struct *pick_task_idle(struct rq *rq)
>  {
> +	scx_update_idle(rq, true);

Thanks a lot for debugging this. Both the analysis and solution make sense
to me. However, as this puts scx_update_idle() in a different place from
other idle handling functions, can you please add a comment explaining why
it needs to be in pick_task_idle() instead of set_next_task_idle()?

Thanks.

-- 
tejun

On Mon, Oct 14, 2024 at 03:12:16PM -1000, Tejun Heo wrote:
> Hello,
> 
> On Tue, Oct 15, 2024 at 12:06:03AM +0200, Andrea Righi wrote:
> > @@ -459,13 +459,13 @@ static void put_prev_task_idle(struct rq *rq, struct task_struct *prev, struct t
> >  static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool first)
> >  {
> >  	update_idle_core(rq);
> > -	scx_update_idle(rq, true);
> >  	schedstat_inc(rq->sched_goidle);
> >  	next->se.exec_start = rq_clock_task(rq);
> >  }
> >  
> >  struct task_struct *pick_task_idle(struct rq *rq)
> >  {
> > +	scx_update_idle(rq, true);
> 
> Thanks a lot for debugging this. Both the analysis and solution make sense
> to me. However, as this puts scx_update_idle() in a different place from
> other idle handling functions, can you please add a comment explaining why
> it needs to be in pick_task_idle() instead of set_next_task_idle()?
> 
> Thanks.

Sure, I'll send a v3 with a proper comment.

Thanks,
-Andrea

> 
> -- 
> tejun