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

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Date:	Thu, 18 Dec 2008 11:56:47 +0100
From:	"Frédéric Weisbecker" <fweisbec@...il.com>
To:	"Thomas Gleixner" <tglx@...utronix.de>
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"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

2008/12/18 Thomas Gleixner <tglx@...utronix.de>:
> On Thu, 18 Dec 2008, Ingo Molnar wrote:
>> * 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?
>
> In reality I have not seen such a problem yet, even on an old real
> slow P1 which I tricked to do highres, but of course if we add such
> time consuming debugs and make it slow enough the system will spend
> all the time running the tick timer :)
>
> We should at least warn once about such a loop.
>
> I'm not sure about the mandatory interval though:
>
> Try the same test with HZ=1000 periodic mode (HIGHRES/NOHZ=off) and I
> bet you see the same problem, just not in hrtimer_interrupt().
>
> Thanks,
>
>        tglx
>

That's right. I hesitate to propagate this patch more globally for all
interrupts....
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