[perfmon2] Profiling FPE's

[Philip M. <ph...@si...>]

Hi Stefane,

An interesting suggested usage of perfmon just came up and I thought  
I'd share it with you. We have need of generating profiles of  
floating point exceptions through perfmon2 sampling, so we can use  
all the tools we've built on it through PAPI. At exception time, the  
kernel emulator has all the relevant and interesting information like  
IP, PID and even the instruction. Better yet, it's precise (no skid).

However, there is no PMU event that directly corresponds to floating  
point exceptions. There certainly could be, however, a software or  
virtual PMD that tracks this event. The emulator itself could keep a  
ring buffer of the above information and that could be then picked up  
by a custom sampling format.

How might such a beastie work in practice? I could have the FP  
emulator bump a per-CPU counter upon exception...but eventually I'd  
want to generate a Perfmon interrupt after a threshold...any ideas?  
This comes back to the issue at hand with the Power series...how to  
handle non-interrupting counters, when you may, in fact, want them to  
trigger something.

Comments?

Phil