Messages in this thread | | | Date | Thu, 18 Dec 2008 12:22:16 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH v2] tracing/function-graph-tracer: prevent from hrtimer interrupt infinite loop |
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* Thomas Gleixner <tglx@linutronix.de> wrote:
> 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().
that would be important to double-check. Frederic, does the system lock up with a periodic 1khz HZ tick just as much? I.e. does the processing of a single timer interrupt take more than 1 milliseconds?
Granted, if the system is too slow to process the system clock, it's not useful.
But that's my point: instead of just randomly disabling functionality until the system gets 'fast enough' to process timer IRQs, how about dynamically and adaptively extending the required minimal timeout between hr-timer IRQs?
That will in essence self-tune the system into some minimally working state - instead of locking it up. Note that such a method would work with any source of timer IRQ slowness - not just tracing.
( And maybe the lockup is somehow hrtimer IRQ induced. If a 1khz clock still works for Frederic then that angle has to be investigated. )
Ingo
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