lkml.org 
[lkml]   [2008]   [Dec]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v2] tracing/function-graph-tracer: prevent from hrtimer interrupt infinite loop
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


\
 
 \ /
  Last update: 2008-12-18 11:51    [W:0.246 / U:0.636 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site