Re: [PATCH 3/3] perf, x86: Add INST_RETIRED.ALL workarounds

[Ingo Molnar <mingo@kernel.org>]

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Mon, Mar 23, 2015 at 10:38:54AM +0100, Ingo Molnar wrote:
> > 
> > * Andi Kleen <andi@firstfloor.org> wrote:
> > 
> > > From: Andi Kleen <ak@linux.intel.com>
> > > 
> > > On Broadwell INST_RETIRED.ALL cannot be used with any period
> > > that doesn't have the lowest 6 bits cleared. And the period
> > > should not be smaller than 128.
> > 
> > Sloppy changelog: a most basic question is not answered by the 
> > changelog: what happens in practice when the period is set to a 
> > smaller value than 128?
> 
> http://www.intel.com/content/dam/www/public/us/en/documents/specification-updates/5th-gen-core-family-spec-update.pdf
> 
> BDM11 and BDM55 (not 57) tell us that the PMU will generate crap output
> if you don't do this. Non-fatal but gibberish.

Should be part of the changelog?

> > > +/*
> > > + * Broadwell:
> > > + * The INST_RETIRED.ALL period always needs to have lowest
> > > + * 6bits cleared (BDM57). It shall not use a period smaller
> > > + * than 100 (BDM11). We combine the two to enforce
> > > + * a min-period of 128.
> > > + */
> > 
> > Sloppy comment: that's not what we do:
> > 
> > > +static unsigned bdw_limit_period(struct perf_event *event, unsigned left)
> > > +{
> > > +	if ((event->hw.config & INTEL_ARCH_EVENT_MASK) ==
> > > +			X86_CONFIG(.event=0xc0, .umask=0x01)) {
> > > +		if (left < 128)
> > > +			left = 128;
> > > +		left &= ~0x3fu;
> > > +	}
> > > +	return left;
> > 
> > We enforce a minimum period of 128 and round the requested period to 
> > 64.
> 
> Not quite, we enforce a min period of 128 but otherwise mask bit0-5, no
> rounding up.

So I did not say rounding up, I meant this sentence:

> > > + *   [...] We combine the two to enforce
> > > + * a min-period of 128.

IMO ambiguously suggests that the result of the combination of the two 
is to enforce a min-period of 128. Would somethin like this:

	          We combine the two to enforce
          a min-period of 128, rounded (down) to multiples of 64.
          The original period is still kept by the core code and is 
          approximated in the long run via these slightly fuzzed 
          hardware-periods.

work with you?

> > I think in this case it would be useful to tooling if we updated 
> > the syscall attribute with the real period value that was used, to 
> > not skew tooling output.
> 
> Seeing how we already have a fuzz of up to sample_period events; we 
> don't know how far into the last period we are when we stop the 
> event, it might have been 1 event away from generating a PMI, this 
> patch doesn't actually add significantly to that.
> 
> Also, the effective period is the one specified, if the requested 
> period < 128 we simply reject the event creation. If its any larger 
> we iterate around the requested sample period with steps of 64 but 
> such that we average out on the requested period. There is no 'real' 
> period to copy back.

Yeah, fair enough.

Thanks,

	Ingo