Re: [PATCH v3] docs/sched_ext: document that cgroup CPU knobs are scheduler-dependent

Hi Tao,

On Mon, Aug 24, 2026 at 05:15:01PM +0800, Tao Cui wrote:
> From: Tao Cui <[email protected]>
> 
> The fair class enforces cpu controller knobs such as cpu.max,
> cpu.weight and cpu.idle in the kernel. sched_ext only passes them to
> the BPF scheduler through the ops.cgroup_set_*() callbacks. Whether
> and how a knob takes effect is up to the loaded scheduler: it may
> implement the corresponding callback partially or not at all. The
> same applies to other knobs like nice levels.
> 
> Document this in the basics section so users and container
> orchestrators know what to expect from a BPF scheduler.
> 
> Signed-off-by: Tao Cui <[email protected]>
> ---
> v2 -> v3: Drop the scheduler list and the nr_throttled example, keep
> the section concise and generic, per review.
> 
> v2: https://lore.kernel.org/r/[email protected]
> 
>  Documentation/scheduler/sched-ext.rst | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/Documentation/scheduler/sched-ext.rst b/Documentation/scheduler/sched-ext.rst
> index 35b550671ca7..b594d93dd6aa 100644
> --- a/Documentation/scheduler/sched-ext.rst
> +++ b/Documentation/scheduler/sched-ext.rst
> @@ -242,6 +242,18 @@ optional. The following modified excerpt is from
>              .name                   = "simple",
>      };
>  
> +Scheduler-Dependent Knobs
> +-------------------------
> +
> +The fair class enforces cpu controller knobs such as ``cpu.max``,
> +``cpu.weight`` and ``cpu.idle`` in the kernel. sched_ext only passes
> +them to the BPF scheduler through ``ops.cgroup_set_weight()``,
> +``ops.cgroup_set_idle()``, ``ops.cgroup_set_bandwidth()`` and friends.

Existing cpu.weight and bandwidth values are delivered via ops.cgroup_init(),
ops.cgroup_set_*() callbacks handle later changes.

Moreover, cpu.idle state is not included in scx_cgroup_init_args, so apparently
BPF schedulers don't receive an initial value (only subsequent writes). This
should be probably fixed separately.

> +Whether and how a knob takes effect is up to the loaded scheduler: it
> +may implement the corresponding callback partially or not at all. The
> +same applies to other knobs like nice levels. When relying on these
> +knobs, check the documentation or source of the loaded scheduler.
> +

"nice levels" can be a bit ambiguous, per-task nice changes are converted to
weights and reported through ops.set_weight(), writes to the cgroup file
cpu.weight.nice are reported through ops.cgroup_set_weight().

Maybe we can rephrase the whole paragraph as following, or something along these
lines:

  The fair-class scheduler enforces CPU controller settings such as cpu.max,
  cpu.weight, and cpu.idle. For sched_ext tasks, the scheduler core communicates
  these settings to the BPF scheduler through ops.cgroup_init() and reports
  subsequent changes through the corresponding ops.cgroup_set_*() callbacks.
  Similarly, per-task nice changes are converted to weights and reported through
  ops.set_weight().

  Each BPF scheduler is responsible for implementing the scheduling semantics of
  these settings and may choose to ignore them. Consult the loaded scheduler's
  documentation before relying on these controls.

Thanks,
-Andrea