linux-kernel - Re: [GIT PULL rcu/next] rcu commits for 2.6.40

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Date:	Tue, 17 May 2011 15:21:45 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Yinghai Lu <yinghai@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org
Subject: Re: [GIT PULL rcu/next] rcu commits for 2.6.40

On Tue, May 17, 2011 at 02:43:48PM +0200, Frederic Weisbecker wrote:
> On Tue, May 17, 2011 at 12:53:49AM -0700, Paul E. McKenney wrote:
> > On Tue, May 17, 2011 at 04:40:03AM +0200, Frederic Weisbecker wrote:
> > > On Mon, May 16, 2011 at 02:24:49PM -0700, Paul E. McKenney wrote:
> > > > On Mon, May 16, 2011 at 02:23:29PM +0200, Ingo Molnar wrote:
> > > > > 
> > > > > * Ingo Molnar <mingo@...e.hu> wrote:
> > > > > 
> > > > > > > In the meantime, would you be willing to try out the patch at 
> > > > > > > https://lkml.org/lkml/2011/5/14/89?  This patch helped out Yinghai in 
> > > > > > > several configurations.
> > > > > > 
> > > > > > Wasn't this the one i tested - or is it a new iteration?
> > > > > > 
> > > > > > I'll try it in any case.
> > > > > 
> > > > > oh, this was a new iteration, mea culpa!
> > > > > 
> > > > > And yes, it solves all problems for me as well. Mind pushing it as a fix? :-)
> > > > 
> > > > ;-)
> > > > 
> > > > Unfortunately, the only reason I can see that it works is (1) there
> > > > is some obscure bug in my code or (2) someone somewhere is failing to
> > > > call irq_exit() on some interrupt-exit path.  Much as I might be tempted
> > > > to paper this one over, I believe that we do need to find whatever the
> > > > underlying bug is.
> > > > 
> > > > Oh, yes, there is option (3) as well: maybe if an interrupt deschedules
> > > > a process, the final irq_exit() is omitted in favor of rcu_enter_nohz()?
> > > > But I couldn't see any evidence of this in my admittedly cursory scan
> > > > of the x86 interrupt-handling code.
> > > > 
> > > > So until I learn differently, I am assuming that each and every
> > > > irq_enter() has a matching call to irq_exit(), and that rcu_enter_nohz()
> > > > is called after the final irq_exit() of a given burst of interrupts.
> > > > 
> > > > If my assumptions are mistaken, please do let me know!
> > > 
> > > So it would be nice to have a trace of the calls to rcu_irq_*() / rcu_*_nohz()
> > > before the unpairing happened.
> > > 
> > > I have tried to reproduce it but couldn't trigger anything.
> > > 
> > > So it would be nice if Yinghai can test the patch below, since he was able
> > > to trigger the warning.
> > > 
> > > This is essentially Paul's patch but with stacktrace of the calls recorded.
> > > Then the whole trace is dumped on the console when one of the WARN_ON_ONCE
> > > sanity check is positive. Beware as the trace will be dumped everytime
> > > WARN_ON_ONCE() is positive. So the first dump is enough, you can ignore the
> > > rest.
> > > 
> > > This requires CONFIG_TRACING. May be a good thing to boot with
> > > "ftrace=nop" parameter, so that ftrace will set up a long enough buffer
> > > to have an interesting trace.
> > 
> > Very cool, thank you!!!  I was going to do something like this next,
> > but given my lack of familiarity with tracing, your patch looks much
> > nicer than mine would have been.
> > 
> > It applies fine on top of tip/core/rcu and builds OK.  I cannot reproduce
> > the problem, either, so I am hoping that either Yinghai or Ingo can
> > run this, and hopefully doing so will provide some enlightenment.
> > 
> > I have pushed this as:
> > 
> > git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-2.6-rcu.git diag.2011.05.16b
> > 
> > I also #ifdefed out the bodies of rcu_nmi_enter() and rcu_nmi_exit()
> > to match the earlier patches.
> > 
> > > PS: the first check in rcu_nmi_enter() doesn't seem to make sense.
> > 
> > Here is what it is doing:
> > 
> > o	rdtp->dynticks_nmi_nesting == 0:
> > 
> > 	Is this is the first-level NMI?  In theory this should always
> > 	be true, but I don't trust NMIs to mask each other.  I have seen
> > 	many systems where NMIs could interrupt other NMIs.
> > 
> > 	The idea is that if we already recorded one level of NMI, we
> > 	had better record them all so we can figure out when we exit
> > 	the last level of NMI handler.
> > 
> > o	atomic_read(&rdtp->dynticks) & 0x1):
> > 
> > 	Did the NMI interrupt a non-dyntick code segment?  If we did,
> > 	then there is no need to tell RCU anything -- RCU is already
> > 	paying attention to this CPU anyway due to the fact that the
> > 	interrupted code segment was not in dyntick mode.
> 
> In fact I was rather referring to your last added check:
> 
> 	if (rdtp->dynticks_nmi_nesting == 0 &&
> -	    (atomic_read(&rdtp->dynticks) & 0x1))
> +	    (atomic_read(&rdtp->dynticks) & 0x1)) {
> +		WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
>  		return;
> +	}

Yes, a bit redundant, but if some other CPU is messing with
rdtp->dynticks, this WARN_ON_ONCE() might catch it.

> > Again, thank you for adding the tracing!
> 
> No problem, I hope it will work as I couldn't test it myself.

Me too!

							Thanx, Paul
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