Re: [PATCH 2/4] sched_ext: Add scx_bpf_task_set_slice() and scx_bpf_task_set_dsq_vtime()

[Changwoo Min]

On 10/7/25 10:51, Tejun Heo wrote:
> With the planned hierarchical scheduler support, sub-schedulers will need to
> be verified for authority before being allowed to modify task->scx.slice and
> task->scx.dsq_vtime. Add scx_bpf_task_set_slice() and
> scx_bpf_task_set_dsq_vtime() which will perform the necessary permission
> checks.

Is it necessary to distinguish between decreasing the slice and
increasing the slice? A BPF scheduler (e.g., LAVD) sets the slice to
zero for passive preemption.

Regards,
Changwoo Min

> Root schedulers can still directly write to these fields, so this doesn't
> affect existing schedulers.
>
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> ---
>   kernel/sched/ext.c                       | 30 ++++++++++++++++++++++++
>   tools/sched_ext/include/scx/common.bpf.h |  2 ++
>   2 files changed, 32 insertions(+)
>
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index fc353b8d69f7..6d76efaaa9b2 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -5833,6 +5833,34 @@ static const struct btf_kfunc_id_set scx_kfunc_set_unlocked = {
>   
>   __bpf_kfunc_start_defs();
>   
> +/**
> + * scx_bpf_task_set_slice - Set task's time slice
> + * @p: task of interest
> + * @slice: time slice to set in nsecs
> + *
> + * Set @p's time slice to @slice. Returns %true on success, %false if the
> + * calling scheduler doesn't have authority over @p.
> + */
> +__bpf_kfunc bool scx_bpf_task_set_slice(struct task_struct *p, u64 slice)
> +{
> +	p->scx.slice = slice;
> +	return true;
> +}
> +
> +/**
> + * scx_bpf_task_set_dsq_vtime - Set task's virtual time for DSQ ordering
> + * @p: task of interest
> + * @vtime: virtual time to set
> + *
> + * Set @p's virtual time to @vtime. Returns %true on success, %false if the
> + * calling scheduler doesn't have authority over @p.
> + */
> +__bpf_kfunc bool scx_bpf_task_set_dsq_vtime(struct task_struct *p, u64 vtime)
> +{
> +	p->scx.dsq_vtime = vtime;
> +	return true;
> +}
> +
>   static void scx_kick_cpu(struct scx_sched *sch, s32 cpu, u64 flags)
>   {
>   	struct rq *this_rq;
> @@ -6638,6 +6666,8 @@ __bpf_kfunc void scx_bpf_events(struct scx_event_stats *events,
>   __bpf_kfunc_end_defs();
>   
>   BTF_KFUNCS_START(scx_kfunc_ids_any)
> +BTF_ID_FLAGS(func, scx_bpf_task_set_slice, KF_RCU);
> +BTF_ID_FLAGS(func, scx_bpf_task_set_dsq_vtime, KF_RCU);
>   BTF_ID_FLAGS(func, scx_bpf_kick_cpu)
>   BTF_ID_FLAGS(func, scx_bpf_dsq_nr_queued)
>   BTF_ID_FLAGS(func, scx_bpf_destroy_dsq)
> diff --git a/tools/sched_ext/include/scx/common.bpf.h b/tools/sched_ext/include/scx/common.bpf.h
> index 06e2551033cb..505231b7b7ae 100644
> --- a/tools/sched_ext/include/scx/common.bpf.h
> +++ b/tools/sched_ext/include/scx/common.bpf.h
> @@ -102,6 +102,8 @@ s32 scx_bpf_pick_any_cpu_node(const cpumask_t *cpus_allowed, int node, u64 flags
>   s32 scx_bpf_pick_any_cpu(const cpumask_t *cpus_allowed, u64 flags) __ksym;
>   bool scx_bpf_task_running(const struct task_struct *p) __ksym;
>   s32 scx_bpf_task_cpu(const struct task_struct *p) __ksym;
> +bool scx_bpf_task_set_slice(struct task_struct *p, u64 slice) __ksym __weak;
> +bool scx_bpf_task_set_dsq_vtime(struct task_struct *p, u64 vtime) __ksym __weak;
>   struct rq *scx_bpf_cpu_rq(s32 cpu) __ksym;
>   struct rq *scx_bpf_locked_rq(void) __ksym;
>   struct task_struct *scx_bpf_cpu_curr(s32 cpu) __ksym __weak;