Re: [RFC PATCH 07/13] sched: Add core wide task selection and scheduling.

[Peter Zijlstra]

On Thu, Apr 16, 2020 at 04:32:28PM -0700, Tim Chen wrote:
> 
> 
> On 4/14/20 6:35 AM, Peter Zijlstra wrote:
> > On Wed, Mar 04, 2020 at 04:59:57PM +0000, vpillai wrote:
> >> +static struct task_struct *
> >> +pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> >> +{
> >> +	struct task_struct *next, *max = NULL;
> >> +	const struct sched_class *class;
> >> +	const struct cpumask *smt_mask;
> >> +	int i, j, cpu;
> >> +	bool need_sync = false;
> > 
> > AFAICT that assignment is superfluous. Also, you violated the inverse
> > x-mas tree.
> > 
> >> +
> >> +	cpu = cpu_of(rq);
> >> +	if (cpu_is_offline(cpu))
> >> +		return idle_sched_class.pick_next_task(rq);
> > 
> > Are we actually hitting this one?
> > 
> 
> I did hit this race when I was testing taking cpu offline and online,
> which prompted the check of cpu being offline.

This is the schedule from the stop task to the idle task I presume,
there should really not be any other. And at that point the rq had
better be empty, so why didn't the normal task selection work?