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

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Date:	Thu, 18 Dec 2008 11:34:59 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	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


* 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.

Thomas, what do you think - would you expect this lockup to happen on 
really slow systems? If yes, is there a way we could avoid it from 
happening - by driving some sort of 'mandatory interval', that is doubled 
in size every time we detect such a bad hrtimer loop?

	Ingo
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