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    SubjectRe: tty^Wrcu/perf lockdep trace.
    On Sat, Oct 05, 2013 at 09:28:02AM -0700, Paul E. McKenney wrote:
    > On Sat, Oct 05, 2013 at 06:05:11PM +0200, Peter Zijlstra wrote:
    > > On Fri, Oct 04, 2013 at 02:25:06PM -0700, Paul E. McKenney wrote:
    > > > > Why
    > > > > do we still have a per-cpu kthread in nocb mode? The idea is that we do
    > > > > not disturb the cpu, right? So I suppose these kthreads get to run on
    > > > > another cpu.
    > > >
    > > > Yep, the idea is that usermode figures out where to run them. Even if
    > > > usermode doesn't do that, this has the effect of getting them to be
    > > > more out of the way of real-time tasks.
    > > >
    > > > > Since its running on another cpu; we get into atomic and memory barriers
    > > > > anyway; so why not keep the logic the same as no-nocb but have another
    > > > > cpu check our nocb cpu's state.
    > > >
    > > > You can do that today by setting rcu_nocb_poll, but that results in
    > > > frequent polling wakeups even when the system is completely idle, which
    > > > is out of the question for the battery-powered embedded guys.
    > >
    > > So its this polling I don't get.. why is the different behaviour
    > > required? And why would you continue polling if the cpus were actually
    > > idle.
    >
    > The idea is to offload the overhead of doing the wakeup from (say)
    > a real-time thread/CPU onto some housekeeping CPU.

    Sure I get that that is the idea; what I don't get is why it needs to
    behave differently depending on NOCB.

    Why does a NOCB thingy need to wake up the kthread far more often?

    > > Is there some confusion between the nr_running==1 extended quiescent
    > > state and the nr_running==0 extended quiescent state?
    >
    > This is independent of the nr_running=1 extended quiescent state. The
    > wakeups only happen when runnning in the kernel. That said, a real-time
    > thread might want both rcu_nocb_poll=y and CONFIG_NO_HZ_FULL=y.

    So there's 3 behaviours?

    - CONFIG_NO_HZ_FULL=n
    - CONFIG_NO_HZ_FULL=y, rcu_nocb_poll=n
    - CONFIG_NO_HZ_FULL=y, rcu_nocb_poll=y

    What I'm trying to understand is why do all those things behave
    differently? For all 3 configs there's kthreads that do the GP advancing
    and can run on different cpus.

    And why does rcu_nocb_poll=y need to be terrible for power usage; surely
    we know when cpus are actually idle and can stop polling them.


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