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

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Date:	Fri, 4 Oct 2013 20:52:39 +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 10:09:54AM -0700, Paul E. McKenney wrote:
> On Fri, Oct 04, 2013 at 06:50:44PM +0200, Peter Zijlstra wrote:
> > On Fri, Oct 04, 2013 at 09:03:52AM -0700, Paul E. McKenney wrote:
> > > The problem exists, but NOCB made it much more probable.  With non-NOCB
> > > kernels, an irq-disabled call_rcu() invocation does a wake_up() only if
> > > there are more than 10,000 callbacks stacked up on the CPU.  With a NOCB
> > > kernel, the wake_up() happens on the first callback.
> > 
> > Oh I see.. so I was hoping this was some NOCB crackbrained damage we
> > could still 'fix'.
> > 
> > And that wakeup is because we moved grace-period advancing into
> > kthreads, right?
> 
> Yep, in earlier kernels we would instead be doing raise_softirq().
> Which would instead wake up ksoftirqd, if I am reading the code
> correctly -- spin_lock_irq() does not affect preempt_count.

I suspect you got lost in the indirection fest; but have a look at
__raw_spin_lock_irqsave(). It does:

	local_irq_save();
	preempt_disable();

> > Probably; so the regular no-NOCB would be easy to work around by
> > providing me a call_rcu variant that never does the wakeup.
> 
> Well, if we can safely, sanely, and reliably defer the wakeup, there is
> no reason not to make plain old call_rcu() do what you need.

Agreed.

> If there
> is no such way to defer the wakeup, then I don't see how to make that
> variant.

Wouldn't it be a simple matter of making __call_rcu_core() return early,
just like it does for irqs_disabled_flags()?

> > NOCB might be a little more difficult; depending on the reason why it
> > needs to do this wakeup on every single invocation; that seems
> > particularly expensive.
> 
> Not on every single invocation, just on those invocations where the list
> is initially empty.  So the first call_rcu() on a CPU whose rcuo kthread
> is sleeping will do a wakeup, but subsequent call_rcu()s will just queue,
> at least until rcuo goes to sleep again.  Which takes awhile, since it
> has to wait for a grace period before invoking that first RCU callback.

So I've not kept up with RCU the last year or so due to circumstance, so
please bear with me ( http://www.youtube.com/watch?v=4sxtHODemi0 ). 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.

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.

That is; I'm fumbling to understand how all this works and needs to be
different.
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