Messages in this thread | | | Date | Sat, 21 Mar 2009 19:18:58 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH 0/5] [GIT PULL] updates for tip/tracing/ftrace |
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* Frederic Weisbecker <fweisbec@gmail.com> wrote:
> > [<ffffffff8020c79d>] return_to_handler+0x0/0x73 > > [<ffffffff8029ea13>] rcu_pending+0x2c/0x5e > > [<ffffffff8020c79d>] return_to_handler+0x0/0x73 > > [<ffffffff8026abef>] update_process_times+0x3c/0x77 > > [<ffffffff8020c79d>] return_to_handler+0x0/0x73 > > [<ffffffff802875dd>] tick_periodic+0x6e/0x70 > > > Still hanging in the timer interrupt. > I guess it makes the timer interrupt servicing too slow and then > once it is serviced, another one is raised. > > But the cause is perhaps more complex > > I think you have had too much hanging of this type. I'm preparing > a fix that checks periodically if the function graph tracer is > spending too much time in an interrupt. > > I guess I could count the number of function executed between the > irq entry and its exit. > > That's the best: if we are hanging in an interrupt, it could be > whatever interrupt and the jiffies could not be progressing so I > can't rely on time but only on number of functions executed. > > May be 10000 calls is a good threshold before killing the function > graph inside an interrupt?
i think the problem isnt even the IRQ handler - but the fact that the (timer) irq handler gets re-triggered - so all we do is processing timer IRQs.
Your patch would detect a timer IRQ hanging - but it would not detect the 'system makes no progress because there's always anoter pending timer IRQ to execute' situation.
So i think we need a "function trace watchdog" - which kills the tracer if we do more than 100,000,000 entries since we started the self-test, or so.
Ingo
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