Re: [PATCH v8 -tip 06/26] sched: Add core wide task selection and scheduling.

[Joel Fernandes]

On Mon, Oct 26, 2020 at 10:31:31AM +0100, Peter Zijlstra wrote:
> On Fri, Oct 23, 2020 at 05:31:18PM -0400, Joel Fernandes wrote:
> > On Fri, Oct 23, 2020 at 09:26:54PM +0200, Peter Zijlstra wrote:
> 
> > > How about this then?
> > 
> > This does look better. It makes sense and I think it will work. I will look
> > more into it and also test it.
> 
> Hummm... Looking at it again I wonder if I can make something like the
> below work.
> 
> (depends on the next patch that pulls core_forceidle into core-wide
> state)
> 
> That would retain the CFS-cgroup optimization as well, for as long as
> there's no cookies around.
> 
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4691,8 +4691,6 @@ pick_next_task(struct rq *rq, struct tas
>  		return next;
>  	}
>  
> -	put_prev_task_balance(rq, prev, rf);
> -
>  	smt_mask = cpu_smt_mask(cpu);
>  
>  	/*
> @@ -4707,14 +4705,25 @@ pick_next_task(struct rq *rq, struct tas
>  	 */
>  	rq->core->core_task_seq++;
>  	need_sync = !!rq->core->core_cookie;
> -
> -	/* reset state */
> -reset:
> -	rq->core->core_cookie = 0UL;
>  	if (rq->core->core_forceidle) {
>  		need_sync = true;
>  		rq->core->core_forceidle = false;
>  	}
> +
> +	if (!need_sync) {
> +		next = __pick_next_task(rq, prev, rf);

This could end up triggering pick_next_task_fair's newidle balancing;

> +		if (!next->core_cookie) {
> +			rq->core_pick = NULL;
> +			return next;
> +		}

.. only to realize here that pick_next_task_fair() that we have to put_prev
the task back as it has a cookie, but the effect of newidle balancing cannot
be reverted.

Would that be a problem as the newly pulled task might be incompatible and
would have been better to leave it alone?

TBH, this is a drastic change and we've done a lot of testing with the
current code and its looking good. I'm a little scared of changing it right
now and introducing regression. Can we maybe do this after the existing
patches are upstream?

thanks,

 - Joel


> +		put_prev_task(next);
> +		need_sync = true;
> +	} else {
> +		put_prev_task_balance(rq, prev, rf);
> +	}
> +
> +	/* reset state */
> +	rq->core->core_cookie = 0UL;
>  	for_each_cpu(i, smt_mask) {
>  		struct rq *rq_i = cpu_rq(i);
>  
> @@ -4744,35 +4752,8 @@ pick_next_task(struct rq *rq, struct tas
>  			 * core.
>  			 */
>  			p = pick_task(rq_i, class, max);
> -			if (!p) {
> -				/*
> -				 * If there weren't no cookies; we don't need to
> -				 * bother with the other siblings.
> -				 */
> -				if (i == cpu && !need_sync)
> -					goto next_class;
> -
> +			if (!p)
>  				continue;
> -			}
> -
> -			/*
> -			 * Optimize the 'normal' case where there aren't any
> -			 * cookies and we don't need to sync up.
> -			 */
> -			if (i == cpu && !need_sync) {
> -				if (p->core_cookie) {
> -					/*
> -					 * This optimization is only valid as
> -					 * long as there are no cookies
> -					 * involved.
> -					 */
> -					need_sync = true;
> -					goto reset;
> -				}
> -
> -				next = p;
> -				goto done;
> -			}
>  
>  			rq_i->core_pick = p;
>