Re: [PATCH sched_ext/for-6.12-fixes] Disable SM_IDLE/rq empty path when scx_enabled

[Tejun Heo <tj@kernel.org>]

Hello,

On Tue, Sep 24, 2024 at 09:10:02AM +0530, K Prateek Nayak wrote:
> >   	prev_state = READ_ONCE(prev->__state);
> >   	if (sched_mode == SM_IDLE) {
> > -		if (!rq->nr_running) {
> > +		/* SCX must consult the BPF scheduler to tell if rq is empty */
> 
> I was wondering if sched_ext case could simply do:
> 
> 		if (scx_enabled())
> 			prev_balance(rq, prev, rf);
> 
> and use "rq->scx.flags" to skip balancing in balance_scx() later when
> __pick_next_task() calls prev_balance() but (and please correct me if
> I'm wrong here) balance_scx() calls balance_one() which can call
> consume_dispatch_q() to pick a task from global / user-defined dispatch
> queue, and in doing so, it does not update "rq->nr_running".

Hmm... would that be a meaningful optimization? prev_balance() calls into
SCX's dispatch path and there can be quite a bit going on there. I'm not
sure whether it'd worth much to save a trip through __pick_next_task().

> I could only see add_nr_running() being called from enqueue_task_scx()
> and this is even before the ext core calls do_enqueue_task() which hooks
> into the bpf layer which makes the decision where the task actually
> goes.
> 
> Is my understanding correct that whichever CPU is the target for the
> enqueue_task_scx() callback initially is the one that accounts the
> enqueue in "rq->nr_running" until the task is dequeued or did I miss
> something?

Whenever a task is dispatched to a local DSQ of a CPU including from
balance_one(), if the task is not on that CPU already,
move_remote_task_to_local_dsq() is called which migrates the task to the
target CPU by deactivating and then re-activating it. As deactivating and
re-activating involves dequeueing and re-enqueueing, rq->running gets
updated accordingly.

Thanks.

-- 
tejun