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

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Message-ID: <20131005160511.GV3081@twins.programming.kicks-ass.net>
Date:	Sat, 5 Oct 2013 18:05:11 +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 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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