linux-kernel - Re: tty^Wrcu/perf lockdep trace.

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Message-ID: <20131005195949.GW3081@twins.programming.kicks-ass.net>
Date:	Sat, 5 Oct 2013 21:59:49 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	gregkh@...uxfoundation.org, peter@...leysoftware.com
Subject: Re: 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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