RE: ia64_do_page_fault shows 19.4% slowdown from notify_die.

["Luck, Tony" <tony.luck@intel.com>]

> 499 nSec/fault ia64_do_page_fault notify_die commented out.
> 501 nSec/fault ia64_do_page_fault with nobody registered.
> 533 nSec/fault notify_die in and just kprobes.
> 596 nSec/fault notify_die in and kdb, kprobes, mca, and xpc loaded.
> 
> The 596 nSec/fault is a 19.4% slowdown.  This is an upcoming OSD beta
> kernel.  It will be representative of what our typical customer will
> have loaded.
> 
> Is this enough justification for breaking notify_die into
> notify_page_fault for the fault path?

I didn't see quite the stability from run to run that your results
suggest.  Running the benchmark five times on the same kernel, I saw
the mean value of the 128 results go from as low as 439 to as high
as 445.  So the difference between commenting in/out the notify_die
call is in the noise.

But comparing the first and last of your results shows that there
is significant slowdown when the notify chain is loaded up with a
ton of stuff, way more than the noise that I see, and I'm glad to see
Anil jumping in to fix this.

-Tony
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