linux-kernel - Re: [PATCH 2/5] perf/x86: add PERF_SAMPLE_SKID

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Date:   Thu, 15 Jun 2017 12:19:27 -0700
From:   Stephane Eranian <eranian@...gle.com>
To:     "Liang, Kan" <kan.liang@...el.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "acme@...hat.com" <acme@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...e.hu" <mingo@...e.hu>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "jolsa@...hat.com" <jolsa@...hat.com>
Subject: Re: [PATCH 2/5] perf/x86: add PERF_SAMPLE_SKID_IP support for X86 PEBS

Hi,

On Thu, Jun 15, 2017 at 9:39 AM, Stephane Eranian <eranian@...gle.com> wrote:
> On Thu, Jun 15, 2017 at 8:40 AM, Liang, Kan <kan.liang@...el.com> wrote:
>>
>>
>>> This patch adds support for SKID_IP to Intel x86 processors in PEBS mode. In
>>> that case, the off-by-1 IP from PEBS is returned in the SKID_IP field.
>>
>> It looks we can only get different skid_ip and ip with :pp event (attr.precise = 2).
>> With the :p event (attr.precise = 1), the skid_ip and ip are the same. Right?
>>
> Correct, because skid_ip would be equal to the pebs->ip.
>
To be more precise, you can always specify PERF_SAMPLE_SKID_IP
with any event or sampling mode. By default, the value saved in each
sample will be the same as the one used for PERF_SAMPLE_IP. On
Intel x86, it only differs with precise mode=2 for events supporting PEBS.


>>> Signed-off-by: Stephane Eranian <eranian@...gle.com>
>>> ---
>>>  arch/x86/events/intel/ds.c | 7 +++++++
>>>  1 file changed, 7 insertions(+)
>>>
>>> diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c index
>>> c6d23ffe422d..ee17de5d6b8d 100644
>>> --- a/arch/x86/events/intel/ds.c
>>> +++ b/arch/x86/events/intel/ds.c
>>> @@ -1169,6 +1169,13 @@ static void setup_pebs_sample_data(struct
>>> perf_event *event,
>>>           x86_pmu.intel_cap.pebs_format >= 1)
>>>               data->addr = pebs->dla;
>>>
>>> +     /*
>>> +      * unmodified, skid IP which is guaranteed to be the next
>>> +      * dyanmic instruction
>>> +      */
>>> +     if (sample_type & PERF_SAMPLE_SKID_IP)
>>> +             data->skid_ip = pebs->ip;
>>> +
>>>       if (x86_pmu.intel_cap.pebs_format >= 2) {
>>>               /* Only set the TSX weight when no memory weight. */
>>>               if ((sample_type & PERF_SAMPLE_WEIGHT) && !fll)
>>> --
>>> 2.13.1.518.g3df882009-goog
>>