Re: [PATCH v2 1/2] sched_ext: Update demo schedulers and selftests to use scx_bpf_task_set_dsq_vtime()

[Andrea Righi <arighi@nvidia.com>]

Hi Cheng-Yang,

On Fri, Mar 13, 2026 at 01:55:26AM +0800, Cheng-Yang Chou wrote:
> Direct writes to p->scx.dsq_vtime are deprecated in favor of
> scx_bpf_task_set_dsq_vtime(). Update scx_simple, scx_flatcg, and
> select_cpu_vtime selftest to use the new kfunc with scale_by_task_weight.

Sorry, that's my mistake, we should use scale_by_task_weight_inverse(), not
scale_by_task_weight() as I suggested in my previous email.

> 
> Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
> ---
>  tools/sched_ext/scx_flatcg.bpf.c                         | 9 +++++----
>  tools/sched_ext/scx_simple.bpf.c                         | 6 ++++--
>  tools/testing/selftests/sched_ext/select_cpu_vtime.bpf.c | 7 +++++--
>  3 files changed, 14 insertions(+), 8 deletions(-)
> 
> diff --git a/tools/sched_ext/scx_flatcg.bpf.c b/tools/sched_ext/scx_flatcg.bpf.c
> index a8a9234bb41e..5cb92b4f1635 100644
> --- a/tools/sched_ext/scx_flatcg.bpf.c
> +++ b/tools/sched_ext/scx_flatcg.bpf.c
> @@ -551,9 +551,10 @@ void BPF_STRUCT_OPS(fcg_stopping, struct task_struct *p, bool runnable)
>  	 * too much, determine the execution time by taking explicit timestamps
>  	 * instead of depending on @p->scx.slice.
>  	 */
> +	u64 delta = scale_by_task_weight(p, SCX_SLICE_DFL - p->scx.slice);

Here.

> +
>  	if (!fifo_sched)
> -		p->scx.dsq_vtime +=
> -			(SCX_SLICE_DFL - p->scx.slice) * 100 / p->scx.weight;
> +		scx_bpf_task_set_dsq_vtime(p, p->scx.dsq_vtime + delta);
>  
>  	taskc = bpf_task_storage_get(&task_ctx, p, 0, 0);
>  	if (!taskc) {
> @@ -822,7 +823,7 @@ s32 BPF_STRUCT_OPS(fcg_init_task, struct task_struct *p,
>  	if (!(cgc = find_cgrp_ctx(args->cgroup)))
>  		return -ENOENT;
>  
> -	p->scx.dsq_vtime = cgc->tvtime_now;
> +	scx_bpf_task_set_dsq_vtime(p, cgc->tvtime_now);
>  
>  	return 0;
>  }
> @@ -924,7 +925,7 @@ void BPF_STRUCT_OPS(fcg_cgroup_move, struct task_struct *p,
>  		return;
>  
>  	delta = time_delta(p->scx.dsq_vtime, from_cgc->tvtime_now);
> -	p->scx.dsq_vtime = to_cgc->tvtime_now + delta;
> +	scx_bpf_task_set_dsq_vtime(p, to_cgc->tvtime_now + delta);
>  }
>  
>  s32 BPF_STRUCT_OPS_SLEEPABLE(fcg_init)
> diff --git a/tools/sched_ext/scx_simple.bpf.c b/tools/sched_ext/scx_simple.bpf.c
> index b456bd7cae77..a61a7df81e49 100644
> --- a/tools/sched_ext/scx_simple.bpf.c
> +++ b/tools/sched_ext/scx_simple.bpf.c
> @@ -121,12 +121,14 @@ void BPF_STRUCT_OPS(simple_stopping, struct task_struct *p, bool runnable)
>  	 * too much, determine the execution time by taking explicit timestamps
>  	 * instead of depending on @p->scx.slice.
>  	 */
> -	p->scx.dsq_vtime += (SCX_SLICE_DFL - p->scx.slice) * 100 / p->scx.weight;
> +	u64 delta = scale_by_task_weight(p, SCX_SLICE_DFL - p->scx.slice);

Here.

> +
> +	scx_bpf_task_set_dsq_vtime(p, p->scx.dsq_vtime + delta);
>  }
>  
>  void BPF_STRUCT_OPS(simple_enable, struct task_struct *p)
>  {
> -	p->scx.dsq_vtime = vtime_now;
> +	scx_bpf_task_set_dsq_vtime(p, vtime_now);
>  }
>  
>  s32 BPF_STRUCT_OPS_SLEEPABLE(simple_init)
> diff --git a/tools/testing/selftests/sched_ext/select_cpu_vtime.bpf.c b/tools/testing/selftests/sched_ext/select_cpu_vtime.bpf.c
> index bfcb96cd4954..63e03fe5d983 100644
> --- a/tools/testing/selftests/sched_ext/select_cpu_vtime.bpf.c
> +++ b/tools/testing/selftests/sched_ext/select_cpu_vtime.bpf.c
> @@ -66,12 +66,15 @@ void BPF_STRUCT_OPS(select_cpu_vtime_running, struct task_struct *p)
>  void BPF_STRUCT_OPS(select_cpu_vtime_stopping, struct task_struct *p,
>  		    bool runnable)
>  {
> -	p->scx.dsq_vtime += (SCX_SLICE_DFL - p->scx.slice) * 100 / p->scx.weight;
> +	u64 delta = scale_by_task_weight(p, SCX_SLICE_DFL - p->scx.slice);

And here.

> +
> +	scx_bpf_task_set_dsq_vtime(p, p->scx.dsq_vtime + delta);
> +
>  }
>  
>  void BPF_STRUCT_OPS(select_cpu_vtime_enable, struct task_struct *p)
>  {
> -	p->scx.dsq_vtime = vtime_now;
> +	scx_bpf_task_set_dsq_vtime(p, vtime_now);
>  }
>  
>  s32 BPF_STRUCT_OPS_SLEEPABLE(select_cpu_vtime_init)
> -- 
> 2.48.1
> 

With s/scale_by_task_weight/scale_by_task_weight_inverse/

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

Thanks,
-Andrea