Re: [PATCH] perf x86: Change SandyBridge and IvyBrige instructionsevent to be precise

From: Jiri Olsa
Date: Wed Feb 20 2013 - 16:53:24 EST

On Wed, Feb 20, 2013 at 10:01:20PM +0100, Stephane Eranian wrote:
> On Wed, Feb 20, 2013 at 9:55 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> > Jiri Olsa <jolsa@xxxxxxxxxx> writes:
> >
> >> Changing the default 'instructions' HW event for SandyBridge and
> >> IvyBrige to use the available precise 'instructions' event.
> >>
> >> The precise event is defined in Intel SDM as:
> >>
> >> INST_RETIRED.ALL - Precise instruction retired event with HW to
> >> reduce effect of PEBS shadow in IP distribution
> >>
> >> for both SandyBridge and IvyBrige micro-archs. It allows to use
> >> precise modifier on 'instructions' event on SandyBridge and
> >> IvyBrige micro-archs which is not possible now.
> >
> There is also the problem that on SNB, PREC_DIST must be
> taken ALONE to be correct. By that, I mean no other counter
> must be active. So I don't think it is a good idea.
> Also PREC_DIST is marked as constrained to counter 2 only.
> That in effect, as Andi pointed out, will make the fixed counter
> for instruction_retired, unused because you've set a umask now.
> I don't think it buys you much. People read too much into what
> PEBS gives you. I can show you examples where PEBS
> is worse than no PEBS.

well, we sort of advertise we have precise event via precise
modifier and it confused me I was not able to use it on

maybe we could export precise alias in sysfs as well which we
could check & use any time there's 'p' modifier specified

not pretty.. meeds more thinking ;-)

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