Re: [patch 1/2] nohz: only wakeup a single target cpu when kicking a task

[Peter Zijlstra]

On Thu, Oct 08, 2020 at 10:59:40AM -0400, Peter Xu wrote:
> On Wed, Oct 07, 2020 at 03:01:52PM -0300, Marcelo Tosatti wrote:
> > +static void tick_nohz_kick_task(struct task_struct *tsk)
> > +{
> > +	int cpu = task_cpu(tsk);
> > +
> > +	/*
> > +	 * If the task concurrently migrates to another cpu,
> > +	 * we guarantee it sees the new tick dependency upon
> > +	 * schedule.
> > +	 *
> > +	 *
> > +	 * set_task_cpu(p, cpu);
> > +	 *   STORE p->cpu = @cpu
> > +	 * __schedule() (switch to task 'p')
> > +	 *   LOCK rq->lock
> > +	 *   smp_mb__after_spin_lock()          STORE p->tick_dep_mask
> > +	 *   tick_nohz_task_switch()            smp_mb() (atomic_fetch_or())
> > +	 *      LOAD p->tick_dep_mask           LOAD p->cpu
> > +	 */
> > +
> > +	preempt_disable();
> 
> Pure question: is preempt_disable() required here?  Same question to
> tick_nohz_full_kick_all().

I think it serializes against hotplug.

> 
> > +	if (cpu_online(cpu))
> > +		tick_nohz_full_kick_cpu(cpu);
> > +	preempt_enable();
> > +}