Re: [PATCH v2 2/5] perf/x86/intel: Record branch type

[Peter Zijlstra]

On Fri, Apr 07, 2017 at 09:48:34AM -0700, Andi Kleen wrote:
> On Fri, Apr 07, 2017 at 05:20:31PM +0200, Peter Zijlstra wrote:
> > On Fri, Apr 07, 2017 at 06:47:43PM +0800, Jin Yao wrote:
> > > Perf already has support for disassembling the branch instruction
> > > and using the branch type for filtering. The patch just records
> > > the branch type in perf_branch_entry.
> > > 
> > > Before recording, the patch converts the x86 branch classification
> > > to common branch classification and compute for checking if the
> > > branches cross 4K or 2MB areas. It's an approximate computing for
> > > crossing 4K page or 2MB page.
> > 
> > The changelog is completely empty of rationale. Why do we care?
> > 
> > Not having the binary is a very bad reason; you can't do much of
> > anything if that's missing.
> 
> It's a somewhat common situation with partially JITed code, if you
> don't have an agent. You can still do a lot of useful things.

Like what? How can you say anything about code you don't have?

> We found it useful to have this extra information during workload
> analysis. Forward conditionals and page crossing jumps
> are indications of frontend problems.

But you already have the exact same information in {to,from}, why would
you need to repackage information already contained?