Re: [RFC PATCH 05/11] sched: Split scheduler execution context

[Joel Fernandes]

Hi,
Just on the yield bits (my opinion):

On Mon, Oct 3, 2022 at 5:45 PM Connor O'Brien <connoro@xxxxxxxxxx> wrote:
>
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>
> Lets define the scheduling context as all the scheduler state in
> task_struct and the execution context as all state required to run the
> task.
>
> Currently both are intertwined in task_struct. We want to logically
> split these such that we can run the execution context of one task
> with the scheduling context of another.
>
> To this purpose introduce rq::proxy to point to the task_struct used
> for scheduler state and preserve rq::curr to denote the execution
> context.
>
> XXX connoro: A couple cases here may need more discussion:
> - sched_yield() and yield_to(): whether we call the sched_class
>   methods for rq->curr or rq->proxy, there seem to be cases where
>   proxy exec could cause a yielding mutex owner to run again
>   immediately. How much of a concern is this?

A task chosen to run again after calling yield is not unprecedented.
>From sched_yield manpage:
"If the calling thread is the only thread in the highest priority list
at that time, it will continue to run after a call to sched_yield()."

[...]
> @@ -8276,6 +8300,11 @@ static void do_sched_yield(void)
>         rq = this_rq_lock_irq(&rf);
>
>         schedstat_inc(rq->yld_count);
> +       /*
> +        * XXX how about proxy exec?
> +        * If a task currently proxied by some other task yields, should we
> +        * apply the proxy or the current yield "behaviour" ?
> +        */
>         current->sched_class->yield_task(rq);

I think we should keep it simple and keep the existing yield behavior.
If there was no priority inversion, and lock owner called yield, why
should it behave any differently?

>
>         preempt_disable();
> @@ -8625,6 +8654,10 @@ EXPORT_SYMBOL(yield);
>   */
>  int __sched yield_to(struct task_struct *p, bool preempt)
>  {
> +       /*
> +        * XXX what about current being proxied?
> +        * Should we use proxy->sched_class methods in this case?
> +        */
>         struct task_struct *curr = current;
>         struct rq *rq, *p_rq;
>         unsigned long flags;

Ditto.

> @@ -8984,7 +9017,7 @@ void __init init_idle(struct task_struct *idle, int cpu)
[...]

Thanks,

 - Joel