netdev - Re: [PATCH diagnostic] Re: HPET regression in 2.6.26 versus 2.6.25 -- RCU problem

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Date:	Mon, 11 Aug 2008 06:17:28 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	David Witbrodt <dawitbro@...global.net>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Yinghai Lu <yhlu.kernel@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH diagnostic] Re: HPET regression in 2.6.26 versus 2.6.25
	-- RCU problem

On Mon, Aug 11, 2008 at 01:38:17PM +0200, Ingo Molnar wrote:
> 
> * Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > And here is the patch.  It is still a bit raw, so the results should 
> > be viewed with some suspicion.  It adds a default-off kernel parameter 
> > CONFIG_RCU_CPU_STALL which must be enabled.
> > 
> > Rather than exponential backoff, it backs off to once per 30 seconds. 
> > My feeling upon thinking on it was that if you have stalled RCU grace 
> > periods for that long, a few extra printk() messages are probably the 
> > least of your worries...
> 
> while this wont debug problems were timer irqs are genuinely stuck for 
> long periods of time, it should find problems with RCU completion logic 
> itself in the presence of correct timer irqs - and the lack of any 
> messages from this debug option should point the finger more firmly in 
> the direction of stalled timer irqs.
> 
> So i find this debug feature rather useful and have applied it to 
> tip/core/rcu (and cleaned it up a bit). I renamed the config option to 
> CONFIG_DEBUG_RCU_STALL to make it more in line with usual debug option 
> names. Lets see whether -tip testing finds any false positives.

Sounds good!

For whatever it is worth, this diagnostic can also locate latency issues
in non-CONFIG_PREEMPT kernels, even when those problems are outside of
preempt_disable() regions.  Latency tracer is of course a better tool
for things -inside- of preempt_disable() regions.

							Thanx, Paul
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