Re: [PATCH sched_ext/for-6.11] sched, sched_ext: Replace scx_next_task_picked() with sched_class->switch_class()

[Peter Zijlstra]

On Thu, Jun 20, 2024 at 12:15:02PM -1000, Tejun Heo wrote:
> scx_next_task_picked() is used by sched_ext to notify the BPF scheduler when
> a CPU is taken away by a task dispatched from a higher priority sched_class
> so that the BPF scheduler can, e.g., punt the task[s] which was running or
> were waiting for the CPU to other CPUs.
> 
> Replace the sched_ext specific hook scx_next_task_picked() with a new
> sched_class operation switch_class().
> 
> The changes are straightforward and the code looks better afterwards.
> However, when !CONFIG_SCHED_CLASS_EXT, this just ends up adding an unused
> hook which is unlikely to be useful to other sched_classes. We can #ifdef
> the op with CONFIG_SCHED_CLASS_EXT but then I'm not sure the code
> necessarily looks better afterwards.
> 
> Please let me know the preference. If adding #ifdef's is preferable, that's
> okay too.
> 
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
>  kernel/sched/core.c  |    5 ++++-
>  kernel/sched/ext.c   |   20 ++++++++++----------
>  kernel/sched/ext.h   |    4 ----
>  kernel/sched/sched.h |    2 ++
>  4 files changed, 16 insertions(+), 15 deletions(-)
> 
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5907,7 +5907,10 @@ restart:
>  	for_each_active_class(class) {
>  		p = class->pick_next_task(rq);
>  		if (p) {
> -			scx_next_task_picked(rq, p, class);
> +			const struct sched_class *prev_class = prev->sched_class;
> +
> +			if (class != prev_class && prev_class->switch_class)
> +				prev_class->switch_class(rq, p);
>  			return p;
>  		}
>  	}

I would much rather see sched_class::pick_next_task() get an extra
argument so that the BPF thing can do what it needs in there and we can
avoid this extra code here.