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

Stephane Eranian
2017-06-15T22:28:10Z

On Thu, Jun 15, 2017 at 1:20 PM, Stephane Eranian <eranian@google.com> wrote:
> 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.

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.