linux-kernel - Re: [PATCH 0/5] perf: add support for capturing skid IP

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Message-ID: <CABPqkBQCbiW=b0kXXCUwh_e1FFTNLgHrA8en2Q1_Ov-W9-NBcQ@mail.gmail.com>
Date:   Thu, 15 Jun 2017 23:52:07 -0700
From:   Stephane Eranian <eranian@...gle.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "mingo@...e.hu" <mingo@...e.hu>,
        "Liang, Kan" <kan.liang@...el.com>, Jiri Olsa <jolsa@...hat.com>
Subject: Re: [PATCH 0/5] perf: add support for capturing skid IP

Andi,

On Thu, Jun 15, 2017 at 4:18 PM, Andi Kleen <ak@...ux.intel.com> wrote:
>> Looking at this approach, the user interface is straightforward,
>> implementation in the x86 code is a bit more hairy because of the way
>> the branch_stack is captured, via the cpuc->lbr_entries. If you assume
>> that SKID_IP cannot be used with any other branch stack mode, then it
>> is easy. It becomes messy if you don't.
>
> That should be fine. After all if you have real LBRs you don't need
> the skid IP.
>
Yes, you still do. This is not the same thing. LBR captures only taken branches.
I care about taken AND non-taken branches and I don't want to sample on a
non-taken event, assuming it is available.

You need to inject the skid ip into the LBR stack somehow. Either
directly in the
registers or in the cpuc->lbr_entries. The injection can only happen
on interrupt
whereas the LBR captures 32 branches. So you'd have the skid ip info
only for the
most recent branch in the stack.