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

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Message-ID: <20131005162802.GP5790@linux.vnet.ibm.com>
Date:	Sat, 5 Oct 2013 09:28:02 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 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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