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

[Ingo Molnar]

* 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?

	Ingo
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