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

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Message-ID: <20131004165044.GV28601@twins.programming.kicks-ass.net>
Date:	Fri, 4 Oct 2013 18:50:44 +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 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?

> I am not too happy about the complexity of deferring, but maybe it is
> the right approach, at least assuming perf isn't going to whack me
> with a timer lock.  ;-)

I'm not too thrilled about trying to move the call_rcu() usage either.

> Any other approaches that I am missing?

Probably; so the regular no-NOCB would be easy to work around by
providing me a call_rcu variant that never does the wakeup.

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.

Man, RCU was so much easier when all it was was a strict per-cpu state
with timer-interrupt driven state machine; non of all this nonsense.
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