[PATCH sched_ext/for-6.16] sched_ext: Call ops.update_idle() after updating builtin idle bits

[PATCH sched_ext/for-6.16] sched_ext: Call ops.update_idle() after updating builtin idle bits

[Tejun Heo]

BPF schedulers that use both builtin CPU idle mechanism and
ops.update_idle() may want to use the latter to create interlocking between
ops.enqueue() and CPU idle transitions so that either ops.enqueue() sees the
idle bit or ops.update_idle() sees the task queued somewhere. This can
prevent race conditions where CPUs go idle while tasks are waiting in DSQs.

For such interlocking to work, ops.update_idle() must be called after
builtin CPU masks are updated. Relocate the invocation. Currently, there are
no ordering requirements on transitions from idle and this relocation isn't
expected to make meaningful differences in that direction.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 kernel/sched/ext_idle.c |   25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)

diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
index ae30de383913..66da03cc0b33 100644
--- a/kernel/sched/ext_idle.c
+++ b/kernel/sched/ext_idle.c
@@ -738,16 +738,6 @@ void __scx_update_idle(struct rq *rq, bool idle, bool do_notify)
 
 	lockdep_assert_rq_held(rq);
 
-	/*
-	 * Trigger ops.update_idle() only when transitioning from a task to
-	 * the idle thread and vice versa.
-	 *
-	 * Idle transitions are indicated by do_notify being set to true,
-	 * managed by put_prev_task_idle()/set_next_task_idle().
-	 */
-	if (SCX_HAS_OP(sch, update_idle) && do_notify && !scx_rq_bypassing(rq))
-		SCX_CALL_OP(sch, SCX_KF_REST, update_idle, rq, cpu_of(rq), idle);
-
 	/*
 	 * Update the idle masks:
 	 * - for real idle transitions (do_notify == true)
@@ -765,6 +755,21 @@ void __scx_update_idle(struct rq *rq, bool idle, bool do_notify)
 	if (static_branch_likely(&scx_builtin_idle_enabled))
 		if (do_notify || is_idle_task(rq->curr))
 			update_builtin_idle(cpu, idle);
+
+	/*
+	 * Trigger ops.update_idle() only when transitioning from a task to
+	 * the idle thread and vice versa.
+	 *
+	 * Idle transitions are indicated by do_notify being set to true,
+	 * managed by put_prev_task_idle()/set_next_task_idle().
+	 *
+	 * This must come after builtin idle update so that BPF schedulers can
+	 * create interlocking between ops.update_idle() and ops.enqueue() -
+	 * either enqueue() sees the idle bit or update_idle() sees the task
+	 * that enqueue() queued.
+	 */
+	if (SCX_HAS_OP(sch, update_idle) && do_notify && !scx_rq_bypassing(rq))
+		SCX_CALL_OP(sch, SCX_KF_REST, update_idle, rq, cpu_of(rq), idle);
 }
 
 static void reset_idle_masks(struct sched_ext_ops *ops)

Re: [PATCH sched_ext/for-6.16] sched_ext: Call ops.update_idle() after updating builtin idle bits

[Andrea Righi]

On Wed, May 21, 2025 at 12:23:06PM -1000, Tejun Heo wrote:
> BPF schedulers that use both builtin CPU idle mechanism and
> ops.update_idle() may want to use the latter to create interlocking between
> ops.enqueue() and CPU idle transitions so that either ops.enqueue() sees the
> idle bit or ops.update_idle() sees the task queued somewhere. This can
> prevent race conditions where CPUs go idle while tasks are waiting in DSQs.
> 
> For such interlocking to work, ops.update_idle() must be called after
> builtin CPU masks are updated. Relocate the invocation. Currently, there are
> no ordering requirements on transitions from idle and this relocation isn't
> expected to make meaningful differences in that direction.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>

Looks good and it also makes sense semantically: potentially any action
performed in ops.update_idle() should be able to override the built-in idle
state, not the other way around.

For example, if we call scx_bpf_test_and_clear_cpu_idle(cpu) from within
ops.update_idle(), I would expect that to effectively "exclude" the CPU
from the idle selection, since the intention is to override the built-in
idle state. But that's not what it's happening if we update the idle
cpumasks after ops.update_idle(). With this patch applied, it works as
expected.

Maybe we should mention this aspect as well in the commit message,
something like this (feel free to rephrase/ignore):

  This also makes the ops.update_idle() behavior semantically consistent:
  any action performed in this callback should be able to override the
  builtin idle state, not the other way around.

In any case:

Reviewed-and-tested-by: Andrea Righi <arighi@nvidia.com>

Thanks,
-Andrea

> ---
>  kernel/sched/ext_idle.c |   25 +++++++++++++++----------
>  1 file changed, 15 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
> index ae30de383913..66da03cc0b33 100644
> --- a/kernel/sched/ext_idle.c
> +++ b/kernel/sched/ext_idle.c
> @@ -738,16 +738,6 @@ void __scx_update_idle(struct rq *rq, bool idle, bool do_notify)
>  
>  	lockdep_assert_rq_held(rq);
>  
> -	/*
> -	 * Trigger ops.update_idle() only when transitioning from a task to
> -	 * the idle thread and vice versa.
> -	 *
> -	 * Idle transitions are indicated by do_notify being set to true,
> -	 * managed by put_prev_task_idle()/set_next_task_idle().
> -	 */
> -	if (SCX_HAS_OP(sch, update_idle) && do_notify && !scx_rq_bypassing(rq))
> -		SCX_CALL_OP(sch, SCX_KF_REST, update_idle, rq, cpu_of(rq), idle);
> -
>  	/*
>  	 * Update the idle masks:
>  	 * - for real idle transitions (do_notify == true)
> @@ -765,6 +755,21 @@ void __scx_update_idle(struct rq *rq, bool idle, bool do_notify)
>  	if (static_branch_likely(&scx_builtin_idle_enabled))
>  		if (do_notify || is_idle_task(rq->curr))
>  			update_builtin_idle(cpu, idle);
> +
> +	/*
> +	 * Trigger ops.update_idle() only when transitioning from a task to
> +	 * the idle thread and vice versa.
> +	 *
> +	 * Idle transitions are indicated by do_notify being set to true,
> +	 * managed by put_prev_task_idle()/set_next_task_idle().
> +	 *
> +	 * This must come after builtin idle update so that BPF schedulers can
> +	 * create interlocking between ops.update_idle() and ops.enqueue() -
> +	 * either enqueue() sees the idle bit or update_idle() sees the task
> +	 * that enqueue() queued.
> +	 */
> +	if (SCX_HAS_OP(sch, update_idle) && do_notify && !scx_rq_bypassing(rq))
> +		SCX_CALL_OP(sch, SCX_KF_REST, update_idle, rq, cpu_of(rq), idle);
>  }
>  
>  static void reset_idle_masks(struct sched_ext_ops *ops)
> 

Re: [PATCH sched_ext/for-6.16] sched_ext: Call ops.update_idle() after updating builtin idle bits

[Changwoo Min]

Thank you, Tejun, for the change!
The change makes sense semantcially.

Acked-by: Changwoo Min <changwoo@igalia.com>

On 5/22/25 07:23, Tejun Heo wrote:
> BPF schedulers that use both builtin CPU idle mechanism and
> ops.update_idle() may want to use the latter to create interlocking between
> ops.enqueue() and CPU idle transitions so that either ops.enqueue() sees the
> idle bit or ops.update_idle() sees the task queued somewhere. This can
> prevent race conditions where CPUs go idle while tasks are waiting in DSQs.
> 
> For such interlocking to work, ops.update_idle() must be called after
> builtin CPU masks are updated. Relocate the invocation. Currently, there are
> no ordering requirements on transitions from idle and this relocation isn't
> expected to make meaningful differences in that direction.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> ---
>   kernel/sched/ext_idle.c |   25 +++++++++++++++----------
>   1 file changed, 15 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
> index ae30de383913..66da03cc0b33 100644
> --- a/kernel/sched/ext_idle.c
> +++ b/kernel/sched/ext_idle.c
> @@ -738,16 +738,6 @@ void __scx_update_idle(struct rq *rq, bool idle, bool do_notify)
>   
>   	lockdep_assert_rq_held(rq);
>   
> -	/*
> -	 * Trigger ops.update_idle() only when transitioning from a task to
> -	 * the idle thread and vice versa.
> -	 *
> -	 * Idle transitions are indicated by do_notify being set to true,
> -	 * managed by put_prev_task_idle()/set_next_task_idle().
> -	 */
> -	if (SCX_HAS_OP(sch, update_idle) && do_notify && !scx_rq_bypassing(rq))
> -		SCX_CALL_OP(sch, SCX_KF_REST, update_idle, rq, cpu_of(rq), idle);
> -
>   	/*
>   	 * Update the idle masks:
>   	 * - for real idle transitions (do_notify == true)
> @@ -765,6 +755,21 @@ void __scx_update_idle(struct rq *rq, bool idle, bool do_notify)
>   	if (static_branch_likely(&scx_builtin_idle_enabled))
>   		if (do_notify || is_idle_task(rq->curr))
>   			update_builtin_idle(cpu, idle);
> +
> +	/*
> +	 * Trigger ops.update_idle() only when transitioning from a task to
> +	 * the idle thread and vice versa.
> +	 *
> +	 * Idle transitions are indicated by do_notify being set to true,
> +	 * managed by put_prev_task_idle()/set_next_task_idle().
> +	 *
> +	 * This must come after builtin idle update so that BPF schedulers can
> +	 * create interlocking between ops.update_idle() and ops.enqueue() -
> +	 * either enqueue() sees the idle bit or update_idle() sees the task
> +	 * that enqueue() queued.
> +	 */
> +	if (SCX_HAS_OP(sch, update_idle) && do_notify && !scx_rq_bypassing(rq))
> +		SCX_CALL_OP(sch, SCX_KF_REST, update_idle, rq, cpu_of(rq), idle);
>   }
>   
>   static void reset_idle_masks(struct sched_ext_ops *ops)
> 
> 

Re: [PATCH sched_ext/for-6.16] sched_ext: Call ops.update_idle() after updating builtin idle bits

[Tejun Heo]

Applied to sched_ext/for-6.16 with commit message update suggested by
Andrea.

------ 8< ------
From 273cc949655c70001778eb0b9e7db993df845912 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Wed, 21 May 2025 12:23:06 -1000
Subject: [PATCH] sched_ext: Call ops.update_idle() after updating builtin idle
 bits

BPF schedulers that use both builtin CPU idle mechanism and
ops.update_idle() may want to use the latter to create interlocking between
ops.enqueue() and CPU idle transitions so that either ops.enqueue() sees the
idle bit or ops.update_idle() sees the task queued somewhere. This can
prevent race conditions where CPUs go idle while tasks are waiting in DSQs.

For such interlocking to work, ops.update_idle() must be called after
builtin CPU masks are updated. Relocate the invocation. Currently, there are
no ordering requirements on transitions from idle and this relocation isn't
expected to make meaningful differences in that direction.

This also makes the ops.update_idle() behavior semantically consistent:
any action performed in this callback should be able to override the
builtin idle state, not the other way around.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-and-tested-by: Andrea Righi <arighi@nvidia.com>
Acked-by: Changwoo Min <changwoo@igalia.com>
---
 kernel/sched/ext_idle.c | 25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)

diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
index ae30de383913..66da03cc0b33 100644
--- a/kernel/sched/ext_idle.c
+++ b/kernel/sched/ext_idle.c
@@ -738,16 +738,6 @@ void __scx_update_idle(struct rq *rq, bool idle, bool do_notify)
 
 	lockdep_assert_rq_held(rq);
 
-	/*
-	 * Trigger ops.update_idle() only when transitioning from a task to
-	 * the idle thread and vice versa.
-	 *
-	 * Idle transitions are indicated by do_notify being set to true,
-	 * managed by put_prev_task_idle()/set_next_task_idle().
-	 */
-	if (SCX_HAS_OP(sch, update_idle) && do_notify && !scx_rq_bypassing(rq))
-		SCX_CALL_OP(sch, SCX_KF_REST, update_idle, rq, cpu_of(rq), idle);
-
 	/*
 	 * Update the idle masks:
 	 * - for real idle transitions (do_notify == true)
@@ -765,6 +755,21 @@ void __scx_update_idle(struct rq *rq, bool idle, bool do_notify)
 	if (static_branch_likely(&scx_builtin_idle_enabled))
 		if (do_notify || is_idle_task(rq->curr))
 			update_builtin_idle(cpu, idle);
+
+	/*
+	 * Trigger ops.update_idle() only when transitioning from a task to
+	 * the idle thread and vice versa.
+	 *
+	 * Idle transitions are indicated by do_notify being set to true,
+	 * managed by put_prev_task_idle()/set_next_task_idle().
+	 *
+	 * This must come after builtin idle update so that BPF schedulers can
+	 * create interlocking between ops.update_idle() and ops.enqueue() -
+	 * either enqueue() sees the idle bit or update_idle() sees the task
+	 * that enqueue() queued.
+	 */
+	if (SCX_HAS_OP(sch, update_idle) && do_notify && !scx_rq_bypassing(rq))
+		SCX_CALL_OP(sch, SCX_KF_REST, update_idle, rq, cpu_of(rq), idle);
 }
 
 static void reset_idle_masks(struct sched_ext_ops *ops)
-- 
2.49.0