Re: [PATCH v15 04/13] task_isolation: add initial support

From: Paul E. McKenney
Date: Tue Sep 27 2016 - 10:48:20 EST

On Tue, Sep 27, 2016 at 04:22:20PM +0200, Frederic Weisbecker wrote:
> On Fri, Sep 02, 2016 at 10:28:00AM -0700, Andy Lutomirski wrote:
> >
> > Unless I'm missing something (which is reasonably likely), couldn't
> > the isolation code just force or require rcu_nocbs on the isolated
> > CPUs to avoid this problem entirely.
> rcu_nocb is already implied by nohz_full. Which means that RCU callbacks
> are offlined outside the nohz_full set of CPUs.

Indeed, at boot time, RCU makes any nohz_full CPU also be a rcu_nocb

> > I admit I still don't understand why the RCU context tracking code
> > can't just run the callback right away instead of waiting however many
> > microseconds in general. I feel like paulmck has explained it to me
> > at least once, but that doesn't mean I remember the answer.
> The RCU context tracking doesn't take care of callbacks. It's only there
> to tell the RCU core whether the CPU runs code that may or may not run
> RCU read side critical sections. This is assumed by "kernel may use RCU,
> userspace can't".

And RCU has to wait for read-side critical sections to complete before
invoking callbacks.

Thanx, Paul