Messages in this thread | | | Date | Thu, 18 Dec 2008 11:56:47 +0100 | From | "Frédéric Weisbecker" <> | Subject | Re: [PATCH v2] tracing/function-graph-tracer: prevent from hrtimer interrupt infinite loop |
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2008/12/18 Thomas Gleixner <tglx@linutronix.de>: > 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 >
That's right. I hesitate to propagate this patch more globally for all interrupts....
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