Date: Fri, 7 Apr 2017 09:48:34 -0700
From: Andi Kleen <ak@linux.intel.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Jin Yao <yao.jin@linux.intel.com>, acme@kernel.org, jolsa@kernel.org,
 mingo@redhat.com, alexander.shishkin@linux.intel.com,
 Linux-kernel@vger.kernel.org, kan.liang@intel.com,
 yao.jin@intel.com, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v2 2/5] perf/x86/intel: Record branch type
Message-ID: <20170407164834.GA4021@tassilo.jf.intel.com>
References: <1491562066-7472-1-git-send-email-yao.jin@linux.intel.com>
 <1491562066-7472-3-git-send-email-yao.jin@linux.intel.com>
 <20170407152031.k2auigfkj7ek4suo@hirez.programming.kicks-ass.net>
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Content-Type: text/plain; charset=us-ascii
In-Reply-To: <20170407152031.k2auigfkj7ek4suo@hirez.programming.kicks-ass.net>

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