Re: [PATCH RT 8/8] sched: Lazy migrate_disable processing

[Scott Wood]

On Tue, 2019-09-17 at 18:50 +0200, Sebastian Andrzej Siewior wrote:
> On 2019-07-27 00:56:38 [-0500], Scott Wood wrote:
> > diff --git a/kernel/cpu.c b/kernel/cpu.c
> > index 885a195dfbe0..0096acf1a692 100644
> > --- a/kernel/cpu.c
> > +++ b/kernel/cpu.c
> > @@ -939,17 +893,34 @@ static int takedown_cpu(unsigned int cpu)
> >  	 */
> >  	irq_lock_sparse();
> >  
> > -#ifdef CONFIG_PREEMPT_RT_FULL
> > -	__write_rt_lock(cpuhp_pin);
> > +#ifdef CONFIG_PREEMPT_RT_BASE
> > +	WARN_ON_ONCE(takedown_cpu_task);
> > +	takedown_cpu_task = current;
> > +
> > +again:
> > +	for (;;) {
> > +		int nr_pinned;
> > +
> > +		set_current_state(TASK_UNINTERRUPTIBLE);
> > +		nr_pinned = cpu_nr_pinned(cpu);
> > +		if (nr_pinned == 0)
> > +			break;
> > +		schedule();
> > +	}
> 
> we used to have cpuhp_pin which ensured that once we own the write lock
> there will be no more tasks that can enter a migrate_disable() section
> on this CPU. It has been placed fairly late to ensure that nothing new
> comes in as part of the shutdown process and that it flushes everything
> out that is still in a migrate_disable() section.
> Now you claim that once the counter reached zero it never increments
> again. I would be happier if there was an explicit check for that :)

I don't claim that.  A check is added in take_cpu_down() to see whether it
went back up, and if so, exit with EAGAIN.  If *that* check succeeds, it
can't go back up because it's in stop machine, and any tasks will get
migrated to another CPU before they can run again.  There's also a WARN in
migrate_tasks() if somehow a migrate-disabled task does get encountered.

> There is no back off and flush mechanism which means on a busy CPU (as
> in heavily lock contended by multiple tasks) this will wait until the
> CPU gets idle again.

Not really any different from the reader-biased rwlock that this replaces...

-Scott