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

[Ingo Molnar <mingo@elte.hu>]

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

> 2008/12/18 Ingo Molnar <mingo@elte.hu>:
> >
> > * Frederic Weisbecker <fweisbec@gmail.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