perf: forcing instructions event to run on Fixed Counter 0

From: Vince Weaver
Date: Mon Apr 15 2013 - 12:09:57 EST



It turns out that perf_event for intel seems to use the INST_RETIRED.ALL
event interchangably with the "Fixed Counter 0" event.

It turns out they are not equivelent. The Fixed Counter 0 event turns out
to be deterministic, while INST_RETIRED.ALL has a bug where it counts
extra events due to hardware interrupts.

Having a user-accessible deterministic instructions event would be really
useful. So is there a way we can specify we want an event to run on Fixed
Counter 0? I think there is code already that does this for Fixed Counter
2 for similar reasons.

For an example of this happening in real life, take the
./retired_instr.all.x86_64 from my deterministic benchmark that
I'll be presenting at the ISPASS conference next week.
(can be found here git://github.com/deater/deterministic.git )

If you run this benchmark with the same event listed 5 times on an Ivy
Bridge machine you get these results, notice the last one is the "proper"
deterministic result and thus the one that ran on Fixed Counter 0.

$ perf stat -e instructions:u,instructions:u,instructions:u,instructions:u,instructions:u ./retired_instr.all.x86_64
...
Performance counter stats for './retired_instr.all.x86_64':

227,010,687 instructions:u # 0.00 insns per cycle
227,010,687 instructions:u # 0.00 insns per cycle
227,010,687 instructions:u # 0.00 insns per cycle
227,010,687 instructions:u # 0.00 insns per cycle
227,000,723 instructions:u # 0.00 insns per cycle

1.902648316 seconds time elapsed

Thanks,

Vince Weaver
vincent.weaver@xxxxxxxxx
http://www.eece.maine.edu/~vweaver/
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/