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

From: "Paul E. McKenney" <[email-masked]>
Date: Mon, 11 Aug 2008 06:17:28 -0700

On Mon, Aug 11, 2008 at 01:38:17PM +0200, Ingo Molnar wrote:
> 
> * Paul E. McKenney <[email-masked]> 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