[PATCH] sched_ext: pass the initial cpu.idle state in scx_cgroup_init_args

[PATCH] sched_ext: pass the initial cpu.idle state in scx_cgroup_init_args

[Tao Cui]

From: Tao Cui <cuitao@kylinos.cn>

scx_cgroup_init_args carries the initial weight and bandwidth control
parameters of a cgroup to ops.cgroup_init(), but not its cpu.idle
state. A cgroup that was already configured idle before the scheduler
was loaded (or before it was onlined under it) is presented as
non-idle, and the BPF scheduler only learns about it if cpu.idle is
written again later.

Add the idle state to scx_cgroup_init_args and fill it in both places
that build the args -- scx_tg_online() for cgroups created under the
scheduler and scx_cgroup_init() for cgroups that already exist when
the scheduler is loaded.

Verified in a VM with a probe scheduler printing the init args: a
cgroup configured cpu.idle=1 before loading shows idle=1 in
ops.cgroup_init(), the default shows 0, and later cpu.idle writes
still come through ops.cgroup_set_idle().

Signed-off-by: Tao Cui <cuitao@kylinos.cn>
---
 kernel/sched/ext/ext.c      | 4 +++-
 kernel/sched/ext/internal.h | 3 +++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index b646711a45fe..a7218314dcfb 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -4764,7 +4764,8 @@ int scx_tg_online(struct task_group *tg)
 				{ .weight = tg->scx.weight,
 				  .bw_period_us = tg->scx.bw_period_us,
 				  .bw_quota_us = tg->scx.bw_quota_us,
-				  .bw_burst_us = tg->scx.bw_burst_us };
+				  .bw_burst_us = tg->scx.bw_burst_us,
+				  .idle = tg->scx.idle };
 
 			ret = SCX_CALL_OP_RET(sch, cgroup_init,
 					      NULL, tg->css.cgroup, &args);
@@ -5185,6 +5186,7 @@ static int scx_cgroup_init(struct scx_sched *sch)
 				.bw_period_us = tg->scx.bw_period_us,
 				.bw_quota_us = tg->scx.bw_quota_us,
 				.bw_burst_us = tg->scx.bw_burst_us,
+				.idle = tg->scx.idle,
 			};
 
 			ret = SCX_CALL_OP_RET(sch, cgroup_init, NULL, css->cgroup, &args);
diff --git a/kernel/sched/ext/internal.h b/kernel/sched/ext/internal.h
index 53e136a47924..aa149a9c29f7 100644
--- a/kernel/sched/ext/internal.h
+++ b/kernel/sched/ext/internal.h
@@ -259,6 +259,9 @@ struct scx_cgroup_init_args {
 	u64			bw_period_us;
 	u64			bw_quota_us;
 	u64			bw_burst_us;
+
+	/* whether the cgroup is configured idle via cpu.idle */
+	bool			idle;
 };
 
 enum scx_cpu_preempt_reason {
-- 
2.43.0

Re: [PATCH] sched_ext: pass the initial cpu.idle state in scx_cgroup_init_args

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The patch fails to initialize the new `idle` field in `struct scx_cgroup_init_args` in the sub-scheduler paths, causing it to implicitly default to false and lose the correct idle state.
--

--- Patch [1]: [PATCH] sched_ext: pass the initial cpu.idle state in scx_cgroup_init_args ---
commit 6b122f2bce26c31e18660129955114867f5a36b0
Author: Tao Cui <cuitao@kylinos.cn>

sched_ext: pass the initial cpu.idle state in scx_cgroup_init_args

This commit adds the idle state to scx_cgroup_init_args and initializes it
in scx_tg_online() and scx_cgroup_init(). This ensures that cgroups already
configured as idle before the scheduler is loaded are correctly presented as
idle to the BPF scheduler.

> diff --git a/kernel/sched/ext/internal.h b/kernel/sched/ext/internal.h
> index 53e136a47924d..aa149a9c29f7b 100644
> --- a/kernel/sched/ext/internal.h
> +++ b/kernel/sched/ext/internal.h
> @@ -259,6 +259,9 @@ struct scx_cgroup_init_args {
>  	u64			bw_period_us;
>  	u64			bw_quota_us;
>  	u64			bw_burst_us;
> +
> +	/* whether the cgroup is configured idle via cpu.idle */
> +	bool			idle;
>  };

[Severity: Medium]
When adding this new field, did we miss updating the struct initialization in
the sub-scheduler paths?

Looking at kernel/sched/ext/sub.c, both scx_cgroup_claim_subtree() and
scx_cgroup_return_subtree() initialize the struct but omit the new field:

kernel/sched/ext/sub.c:scx_cgroup_claim_subtree() {
    ...
		struct scx_cgroup_init_args args = {
			.weight = tg->scx.weight,
			.bw_period_us = tg->scx.bw_period_us,
			.bw_quota_us = tg->scx.bw_quota_us,
			.bw_burst_us = tg->scx.bw_burst_us,
		};
    ...
}

The exact same pattern also exists in scx_cgroup_return_subtree().

Without explicitly setting the idle field in these paths, wouldn't it
implicitly default to false, causing cgroups to incorrectly lose their idle
state when a sub-scheduler is enabled or disabled?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260824133954.561956-1-cui.tao@linux.dev?part=1

Re: [PATCH] sched_ext: pass the initial cpu.idle state in scx_cgroup_init_args

[Andrea Righi]

Hi Tao,

On Mon, Aug 24, 2026 at 09:39:54PM +0800, Tao Cui wrote:
> From: Tao Cui <cuitao@kylinos.cn>
> 
> scx_cgroup_init_args carries the initial weight and bandwidth control
> parameters of a cgroup to ops.cgroup_init(), but not its cpu.idle
> state. A cgroup that was already configured idle before the scheduler
> was loaded (or before it was onlined under it) is presented as
> non-idle, and the BPF scheduler only learns about it if cpu.idle is
> written again later.
> 
> Add the idle state to scx_cgroup_init_args and fill it in both places
> that build the args -- scx_tg_online() for cgroups created under the
> scheduler and scx_cgroup_init() for cgroups that already exist when
> the scheduler is loaded.
> 
> Verified in a VM with a probe scheduler printing the init args: a
> cgroup configured cpu.idle=1 before loading shows idle=1 in
> ops.cgroup_init(), the default shows 0, and later cpu.idle writes
> still come through ops.cgroup_set_idle().
> 
> Signed-off-by: Tao Cui <cuitao@kylinos.cn>
> ---
>  kernel/sched/ext/ext.c      | 4 +++-
>  kernel/sched/ext/internal.h | 3 +++
>  2 files changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
> index b646711a45fe..a7218314dcfb 100644
> --- a/kernel/sched/ext/ext.c
> +++ b/kernel/sched/ext/ext.c
> @@ -4764,7 +4764,8 @@ int scx_tg_online(struct task_group *tg)
>  				{ .weight = tg->scx.weight,
>  				  .bw_period_us = tg->scx.bw_period_us,
>  				  .bw_quota_us = tg->scx.bw_quota_us,
> -				  .bw_burst_us = tg->scx.bw_burst_us };
> +				  .bw_burst_us = tg->scx.bw_burst_us,
> +				  .idle = tg->scx.idle };
>  
>  			ret = SCX_CALL_OP_RET(sch, cgroup_init,
>  					      NULL, tg->css.cgroup, &args);
> @@ -5185,6 +5186,7 @@ static int scx_cgroup_init(struct scx_sched *sch)
>  				.bw_period_us = tg->scx.bw_period_us,
>  				.bw_quota_us = tg->scx.bw_quota_us,
>  				.bw_burst_us = tg->scx.bw_burst_us,
> +				.idle = tg->scx.idle,
>  			};
>  
>  			ret = SCX_CALL_OP_RET(sch, cgroup_init, NULL, css->cgroup, &args);
> diff --git a/kernel/sched/ext/internal.h b/kernel/sched/ext/internal.h
> index 53e136a47924..aa149a9c29f7 100644
> --- a/kernel/sched/ext/internal.h
> +++ b/kernel/sched/ext/internal.h
> @@ -259,6 +259,9 @@ struct scx_cgroup_init_args {
>  	u64			bw_period_us;
>  	u64			bw_quota_us;
>  	u64			bw_burst_us;
> +
> +	/* whether the cgroup is configured idle via cpu.idle */
> +	bool			idle;
>  };

It looks like there are two more places in kernel/sched/ext/sub.c,
scx_cgroup_claim_subtree() and scx_cgroup_return_subtree(), they should also
initialize idle. Otherwise the field defaults to false and an idle cgroup would
be presented as non-idle when it's handed to a sub-scheduler.

Thanks,
-Andrea

Re: [PATCH] sched_ext: pass the initial cpu.idle state in scx_cgroup_init_args

[Tao Cui]

Hello, Andrea

在 2026/8/24 22:11, Andrea Righi 写道:
> Hi Tao,
> 
> On Mon, Aug 24, 2026 at 09:39:54PM +0800, Tao Cui wrote:
>> From: Tao Cui <cuitao@kylinos.cn>
>>
>> scx_cgroup_init_args carries the initial weight and bandwidth control
>> parameters of a cgroup to ops.cgroup_init(), but not its cpu.idle
>> state. A cgroup that was already configured idle before the scheduler
>> was loaded (or before it was onlined under it) is presented as
>> non-idle, and the BPF scheduler only learns about it if cpu.idle is
>> written again later.
>>
>> Add the idle state to scx_cgroup_init_args and fill it in both places
>> that build the args -- scx_tg_online() for cgroups created under the
>> scheduler and scx_cgroup_init() for cgroups that already exist when
>> the scheduler is loaded.
>>
>> Verified in a VM with a probe scheduler printing the init args: a
>> cgroup configured cpu.idle=1 before loading shows idle=1 in
>> ops.cgroup_init(), the default shows 0, and later cpu.idle writes
>> still come through ops.cgroup_set_idle().
>>
>> Signed-off-by: Tao Cui <cuitao@kylinos.cn>
>> ---
>>  kernel/sched/ext/ext.c      | 4 +++-
>>  kernel/sched/ext/internal.h | 3 +++
>>  2 files changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
>> index b646711a45fe..a7218314dcfb 100644
>> --- a/kernel/sched/ext/ext.c
>> +++ b/kernel/sched/ext/ext.c
>> @@ -4764,7 +4764,8 @@ int scx_tg_online(struct task_group *tg)
>>  				{ .weight = tg->scx.weight,
>>  				  .bw_period_us = tg->scx.bw_period_us,
>>  				  .bw_quota_us = tg->scx.bw_quota_us,
>> -				  .bw_burst_us = tg->scx.bw_burst_us };
>> +				  .bw_burst_us = tg->scx.bw_burst_us,
>> +				  .idle = tg->scx.idle };
>>  
>>  			ret = SCX_CALL_OP_RET(sch, cgroup_init,
>>  					      NULL, tg->css.cgroup, &args);
>> @@ -5185,6 +5186,7 @@ static int scx_cgroup_init(struct scx_sched *sch)
>>  				.bw_period_us = tg->scx.bw_period_us,
>>  				.bw_quota_us = tg->scx.bw_quota_us,
>>  				.bw_burst_us = tg->scx.bw_burst_us,
>> +				.idle = tg->scx.idle,
>>  			};
>>  
>>  			ret = SCX_CALL_OP_RET(sch, cgroup_init, NULL, css->cgroup, &args);
>> diff --git a/kernel/sched/ext/internal.h b/kernel/sched/ext/internal.h
>> index 53e136a47924..aa149a9c29f7 100644
>> --- a/kernel/sched/ext/internal.h
>> +++ b/kernel/sched/ext/internal.h
>> @@ -259,6 +259,9 @@ struct scx_cgroup_init_args {
>>  	u64			bw_period_us;
>>  	u64			bw_quota_us;
>>  	u64			bw_burst_us;
>> +
>> +	/* whether the cgroup is configured idle via cpu.idle */
>> +	bool			idle;
>>  };
> 
> It looks like there are two more places in kernel/sched/ext/sub.c,
> scx_cgroup_claim_subtree() and scx_cgroup_return_subtree(), they should also
> initialize idle. Otherwise the field defaults to false and an idle cgroup would
> be presented as non-idle when it's handed to a sub-scheduler.
> 

Right, I missed those two. v2 fills .idle in all four places that
build scx_cgroup_init_args, including both sub-scheduler handover
paths in sub.c. The main paths are verified in a VM; the sub.c ones
are compile-tested only.

Thanks,
Tao> Thanks,
> -Andrea