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

Stephane Eranian
2017-06-15T20:20:03Z

On Thu, Jun 15, 2017 at 1:02 PM, Andi Kleen <ak@linux.intel.com> wrote:
> On Thu, Jun 15, 2017 at 12:35:39PM -0700, Stephane Eranian wrote:
>> On Thu, Jun 15, 2017 at 10:23 AM, Andi Kleen <ak@linux.intel.com> wrote:
>> > On Thu, Jun 15, 2017 at 09:44:07AM -0700, Stephane Eranian wrote:
>> >> On Thu, Jun 15, 2017 at 8:10 AM, Andi Kleen <ak@linux.intel.com> wrote:
>> >> > On Thu, Jun 15, 2017 at 06:56:24AM -0700, Stephane Eranian wrote:
>> >> >> This patchs adds a new sample record type called
>> >> >> PERF_SAMPLE_SKID_IP. The goal is to record
>> >> >> the unmodified interrupted instruction pointer (IP) as seen by
>> >> >> the kernel and reflected in the machine state.
>> >> >
>> >> > Patches look reasonable for me.
>> >> >
>> >> > If you only cared about branches it would be more natural to model
>> >> > it like a 1 entry LBR. That would make a lot more tooling work
>> >> > automatically.
>> >> >
>> >> You'd still have to modify tooling to present correct column headers.
>> >
>> > Why? It's from/to?
>> >
>> Ah, yes you are right, but it is not clear to me how you would specify
>> this cleanly with the interface.
>> Especially in the case where this could be used for non-branch instructions.
>
> Generally the skid ip is only interesting for instructions that have some kind of
> control flow change, or an exception/interrupt.
>
True, right now I found use for the control flow changes, but any PEBS enabled
event + precise=2 could potentially be useful.
> So if it's interesting the headers would be correct.
>
> Actually it's even correct for non control flow change, because the IP
> moves from "FROM" to "TO" ...
>
Sure.
But I am not too fond of tying this to a branch abstraction. You'd
have to express
this with the perf_events interface.To trigger LBR-style hardware, you'd have
to define a new PERF_SAMPLE_BRANCH_SKID_IP of some sort and then you
tie this to a branch kind of sampling feature:

$ perf record -j skid_ip -e BR_INST_RETIRED.CONDITIONAL:pp

Is what I think you are thinking about.