Re: perf: forcing instructions event to run on Fixed Counter 0

From: Stephane Eranian
Date: Mon Apr 15 2013 - 12:21:50 EST

On Mon, Apr 15, 2013 at 6:13 PM, Vince Weaver <vincent.weaver@xxxxxxxxx> wrote:
> It turns out that perf_event for intel seems to use the INST_RETIRED.ALL
> event interchangably with the "Fixed Counter 0" event.
Yes, it does.

> 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.
Never heard of that problem. I know there was another problem due to leaking
during priv level transitions. It would be take a few instr or cycles to realize
you were not in user level any more when doing event:u.

Interrupt should impact fixed and generic counters the same way.

> 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.
No. Fixed counter 2 (ref-cycles) is a different reason. It's because it measures
an event that does not exist on generic counters: unhalted_reference_cycles.

> 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:// )
> 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.
Are you sure that the 5th event stayed in fixed counter 0 all along?

> $ 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
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