Re: [PATCH 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 Thu, Mar 12, 2026 at 12:20:00PM +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.
> 
> Use bpf_ksym_exists() to fall back to direct assignment on older kernels
> that don't have the new kfunc, preserving backwards compatibility.
> 
> Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
> ---
>  tools/sched_ext/scx_flatcg.bpf.c              | 21 ++++++++++++++-----
>  tools/sched_ext/scx_simple.bpf.c              | 12 +++++++++--
>  .../sched_ext/select_cpu_vtime.bpf.c          | 13 ++++++++++--
>  3 files changed, 37 insertions(+), 9 deletions(-)
> 
> diff --git a/tools/sched_ext/scx_flatcg.bpf.c b/tools/sched_ext/scx_flatcg.bpf.c
> index a8a9234bb41e..27d99bb1e60f 100644
> --- a/tools/sched_ext/scx_flatcg.bpf.c
> +++ b/tools/sched_ext/scx_flatcg.bpf.c
> @@ -551,9 +551,14 @@ 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.
>  	 */
> -	if (!fifo_sched)
> -		p->scx.dsq_vtime +=
> -			(SCX_SLICE_DFL - p->scx.slice) * 100 / p->scx.weight;
> +	u64 delta = (SCX_SLICE_DFL - p->scx.slice) * 100 / p->scx.weight;

While at it, can we use:

 u64 delta = scale_by_task_weight(p, SCX_SLICE_DFL - p->scx.slice);

> +
> +	if (!fifo_sched) {
> +		if (bpf_ksym_exists(scx_bpf_task_set_dsq_vtime___new))
> +			scx_bpf_task_set_dsq_vtime___new(p, p->scx.dsq_vtime + delta);
> +		else
> +			p->scx.dsq_vtime += delta;
> +	}

No, let's just use:

 scx_bpf_task_set_dsq_vtime(p, p->scx.dsq_vtime + delta);

It's already doing the bpf_ksym_exists() logic internally, see
tools/sched_ext/include/scx/compat.bpf.h.

>  
>  	taskc = bpf_task_storage_get(&task_ctx, p, 0, 0);
>  	if (!taskc) {
> @@ -822,7 +827,10 @@ 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;
> +	if (bpf_ksym_exists(scx_bpf_task_set_dsq_vtime___new))
> +		scx_bpf_task_set_dsq_vtime___new(p, cgc->tvtime_now);
> +	else
> +		p->scx.dsq_vtime = cgc->tvtime_now;

Ditto.

>  
>  	return 0;
>  }
> @@ -924,7 +932,10 @@ 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;
> +	if (bpf_ksym_exists(scx_bpf_task_set_dsq_vtime___new))
> +		scx_bpf_task_set_dsq_vtime___new(p, to_cgc->tvtime_now + delta);
> +	else
> +		p->scx.dsq_vtime = to_cgc->tvtime_now + delta;

Ditto.

>  }
>  
>  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..61d3bcf54ce7 100644
> --- a/tools/sched_ext/scx_simple.bpf.c
> +++ b/tools/sched_ext/scx_simple.bpf.c
> @@ -121,12 +121,20 @@ 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 = (SCX_SLICE_DFL - p->scx.slice) * 100 / p->scx.weight;
> +
> +	if (bpf_ksym_exists(scx_bpf_task_set_dsq_vtime___new))
> +		scx_bpf_task_set_dsq_vtime___new(p, p->scx.dsq_vtime + delta);
> +	else
> +		p->scx.dsq_vtime += delta;

Ditto.

>  }
>  
>  void BPF_STRUCT_OPS(simple_enable, struct task_struct *p)
>  {
> -	p->scx.dsq_vtime = vtime_now;
> +	if (bpf_ksym_exists(scx_bpf_task_set_dsq_vtime___new))
> +		scx_bpf_task_set_dsq_vtime___new(p, vtime_now);
> +	else
> +		p->scx.dsq_vtime = vtime_now;

Ditto.

>  }
>  
>  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..17ed515c3e25 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 +66,21 @@ 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 = (SCX_SLICE_DFL - p->scx.slice) * 100 / p->scx.weight;

Ditto (scale_by_task_weight).

> +
> +	if (bpf_ksym_exists(scx_bpf_task_set_dsq_vtime___new))
> +		scx_bpf_task_set_dsq_vtime___new(p, p->scx.dsq_vtime + delta);
> +	else
> +		p->scx.dsq_vtime += delta;
> +

Ditto.

>  }
>  
>  void BPF_STRUCT_OPS(select_cpu_vtime_enable, struct task_struct *p)
>  {
> -	p->scx.dsq_vtime = vtime_now;
> +	if (bpf_ksym_exists(scx_bpf_task_set_dsq_vtime___new))
> +		scx_bpf_task_set_dsq_vtime___new(p, vtime_now);
> +	else
> +		p->scx.dsq_vtime = vtime_now;

Ditto.

>  }
>  
>  s32 BPF_STRUCT_OPS_SLEEPABLE(select_cpu_vtime_init)
> -- 
> 2.48.1
> 

Thanks,
-Andrea