Re: [PATCH 03/22] perf/x86/intel: Support adaptive PEBSv4

[Andi Kleen]

On Tue, Mar 19, 2019 at 03:47:48PM +0100, Peter Zijlstra wrote:
> On Mon, Mar 18, 2019 at 02:41:25PM -0700, kan.liang@xxxxxxxxxxxxxxx wrote:
> > From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> > 
> > Adaptive PEBS is a new way to report PEBS sampling information. Instead
> > of a fixed size record for all PEBS events it allows to configure the
> > PEBS record to only include the information needed. Events can then opt
> > in to use such an extended record, or stay with a basic record which
> > only contains the IP.
> > 
> > The major new feature is to support LBRs in PEBS record.
> > This allows (much faster) large PEBS, while still supporting callstacks
> > through callstack LBR. 
> 
> Does it also allow normal LBR usage? Or does it have to be callstacks?

It allows normal LBR too. But I would expect callstack to be the most
common one.  As long as you set a period you can get multi-record
PEBS with -g, which has a lot lower lower overhead than using
PMIs.

Eventually I hope we can even make multi-record PEBS
work in frequency mode by averaging the frequency over multiple
records.


> >  	hwc->config |= ARCH_PERFMON_EVENTSEL_INT;
> > +	hwc->config &= ~ICL_EVENTSEL_ADAPTIVE;
> 
> Just curious; the way I read the SDM, we could leave this set, is that
> correct?

It needs to be cleared to get the basic record (which should be
a common case)

-Andi