Re: [PATCH v2] tracing/function-graph-tracer: prevent from hrtimerinterrupt infinite loop

[Thomas Gleixner]

On Thu, 18 Dec 2008, Ingo Molnar wrote:
> * Frederic Weisbecker <fweisbec@xxxxxxxxx> 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
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