Re: [PATCH RFC tip/core/rcu] Eliminate deadlock between CPU hotplug and expedited grace periods

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Sep 03, 2014 at 08:03:06AM -0700, Paul E. McKenney wrote:
> > > Normal RCU grace periods avoid this by synchronizing on a lock acquired by
> > > the RCU CPU-hotplug notifiers, but this does not work for the expedited
> > > grace periods because the outgoing CPU can be running random tasks for
> > > quite some time after RCU's notifier executes.  So the fix is just to
> > > drop back to a normal grace period when there is a CPU-hotplug operation
> > > in progress.
> > 
> > So why are we 'normally' doing an expedited call here anyhow? 
> 
> Presumably because they set either the boot parameter or
> the sysfs variable that causes synchronize_sched() to so
> synchronize_sched_expedited().

That's not a why but a how. Why does that option exist, why are we doing
this?

I cannot actually find a sysfs variable that controls this though; only
the rcu_pm_notifier. It seems to favour doing an expedited call when
suspending on 'small' machines.

> > But those are not within hotplug bits. Also weren't we removing them? I
> > thought we didn't appreciate spraying IPIs like they do?
> 
> I hadn't heard anything about removing them, but making the
> expedited primitives a bit less IPI-happy is on my list.

I had some recollections of removing a fair number of expedited calls,
but its was a long while ago so what do I know ;-)

Making them less IPI happy would be good indeed.