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

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Date:   Fri, 7 Apr 2017 09:48:34 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jin Yao <yao.jin@...ux.intel.com>, acme@...nel.org,
        jolsa@...nel.org, mingo@...hat.com,
        alexander.shishkin@...ux.intel.com, Linux-kernel@...r.kernel.org,
        kan.liang@...el.com, yao.jin@...el.com,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 2/5] perf/x86/intel: Record branch type

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.

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

-Andi