linux-kernel - Re: [PATCH v2] tracing/function-graph-tracer: prevent from hrtimer interrupt infinite loop

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Date:	Thu, 18 Dec 2008 12:16:12 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Frédéric Weisbecker <fweisbec@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] tracing/function-graph-tracer: prevent from hrtimer
	interrupt infinite loop


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

> 2008/12/18 Ingo Molnar <mingo@...e.hu>:
> >
> > * Frederic Weisbecker <fweisbec@...il.com> wrote:
> >
> >> Impact: fix a system hang on slow systems
> >>
> >> While testing the function graph tracer on VirtualBox, I had a system hang
> >> immediatly after enabling the tracer.
> >>
> >> If hrtimer is enabled on kernel, a slow system can spend too much time
> >> during tracing the hrtimer_interrupt which will do eternal loops,
> >> assuming it always have to retry its process because too much time
> >> elapsed during its time update. Now we provide a feature which lurks at
> >> the number of retries on hrtimer_interrupt. After 10 retries, the
> >> function graph tracer will definetly stop its tracing.
> >
> > hm, i dont really like this solution - it just works around the problem by
> > 'speeding up' the system. If we have a _real_ slow system, there's no such
> > way for us to speed it up.
> 
> 
> It doesn' speed up the system actually. That's not the goal of this 
> patch.

i meant that abstractly. The "solution" here is that your patch turns off 
the function-graph-tracer. The practical effect of that is: the system 
gets much faster at processing hrtimer IRQs and effectively "speeds up".

ok?

	Ingo
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