Messages in this thread | | | Date | Thu, 18 Dec 2008 11:48:13 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [PATCH v2] tracing/function-graph-tracer: prevent from hrtimer interrupt infinite loop |
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On Thu, 18 Dec 2008, Ingo Molnar wrote: > * 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. > > 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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