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Re: Question: kprobe fault handler
On Tue, May 10, 2005 at 04:01:58PM -0700, Keshavamurthy Anil S wrote:
> On Tue, May 10, 2005 at 04:34:42PM -0400, Ananth N Mavinakayanahalli wrote:
> > On Tue, May 10, 2005 at 12:35:48PM -0700, Keshavamurthy, Anil S wrote:
>
> > I think the best explaination as to why that hook is needed in the
> > fast-path was given by Dave Miller. Please see:
> >
> > http://marc.theaimsgroup.com/?l=linux-kernel&m=109898820219611&w=2
> Thanks for the link.
>
> >
> > The overhead is: notifier_call_chain() -> kprobe_running() check in
> > kprobe_exceptions_notify(). And, if any other debugger/utility
> > (eg., kgdb) is using the die_notifier, it will also be invoked.
>
> So you are saying that for every page fault, the overhead is
> notifier_call_chain()->kprobe_exception_notify()->kprobe_running(),
> I agree this is very small when you don't have any kprobes registered.
>
> Imagine a case where 1000 Kprobes are registered and in this
> case for every pagefault, notifier_call_chain() has to loop for 1000
> times, this seems to me an overkill. Please correct me if I am wrong.
No, notifier_call_chain() won't notify _every_ kprobe. Remember,
kprobe_running() will return true _only_ if kprobe_cpu is set, ie., only
if you are processing a kprobe. So, the notifier call is made only once,
irrespective of the number of kprobes registered.
Ananth