linux-kernel - Re: [PATCH 0/5] [GIT PULL] updates for tip/tracing/ftrace

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Date:	Sat, 21 Mar 2009 19:18:58 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 0/5] [GIT PULL] updates for tip/tracing/ftrace


* Frederic Weisbecker <fweisbec@...il.com> wrote:

> >  [<ffffffff8020c79d>] return_to_handler+0x0/0x73
> >  [<ffffffff8029ea13>] rcu_pending+0x2c/0x5e
> >  [<ffffffff8020c79d>] return_to_handler+0x0/0x73
> >  [<ffffffff8026abef>] update_process_times+0x3c/0x77
> >  [<ffffffff8020c79d>] return_to_handler+0x0/0x73
> >  [<ffffffff802875dd>] tick_periodic+0x6e/0x70
> 
> 
> Still hanging in the timer interrupt.
> I guess it makes the timer interrupt servicing too slow and then
> once it is serviced, another one is raised.
> 
> But the cause is perhaps more complex
> 
> I think you have had too much hanging of this type. I'm preparing 
> a fix that checks periodically if the function graph tracer is 
> spending too much time in an interrupt.
> 
> I guess I could count the number of function executed between the 
> irq entry and its exit.
> 
> That's the best: if we are hanging in an interrupt, it could be 
> whatever interrupt and the jiffies could not be progressing so I 
> can't rely on time but only on number of functions executed.
> 
> May be 10000 calls is a good threshold before killing the function 
> graph inside an interrupt?

i think the problem isnt even the IRQ handler - but the fact that 
the (timer) irq handler gets re-triggered - so all we do is 
processing timer IRQs.

Your patch would detect a timer IRQ hanging - but it would not 
detect the 'system makes no progress because there's always anoter 
pending timer IRQ to execute' situation.

So i think we need a "function trace watchdog" - which kills the 
tracer if we do more than 100,000,000 entries since we started the 
self-test, or so.

	Ingo
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