Messages in this thread | | | Date | Sat, 5 Oct 2013 18:05:11 +0200 | From | Peter Zijlstra <> | Subject | Re: tty^Wrcu/perf lockdep trace. |
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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.
Is there some confusion between the nr_running==1 extended quiescent state and the nr_running==0 extended quiescent state?
Now, none of this solves the issue at hand because event the 'regular' no-nocb rcu mode has this issue of needing to wake kthreads, but I'd like to get a better understanding of why nocb mode is as it is.
I've seen you've since send a few more emails; I might find some of the answers in there. Let me go read the :-)
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