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 CPU. > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
