Messages in this thread | | | Date | Sat, 5 Oct 2013 09:28:02 -0700 | From | "Paul E. McKenney" <> | Subject | Re: tty^Wrcu/perf lockdep trace. |
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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.
> 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.
> 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 :-)
I -think- I have solved it, but much testing and review will of course be required. And fixing last night's test failures...
Thanx, Paul
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