Messages in this thread | | | Date | Sat, 5 Oct 2013 21:59:49 +0200 | From | Peter Zijlstra <> | Subject | Re: tty^Wrcu/perf lockdep trace. |
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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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