Re: [PATCH V2 6/7] perf, x86: Use LBR call stack to get user callchain

["Yan, Zheng" <zheng.z.yan@intel.com>]

On 10/24/2012 07:47 PM, Stephane Eranian wrote:
> On Wed, Oct 24, 2012 at 1:23 PM, Yan, Zheng <zheng.z.yan@intel.com> wrote:
>> On 10/24/2012 04:57 PM, Stephane Eranian wrote:
>>> On Wed, Oct 24, 2012 at 7:59 AM, Yan, Zheng <zheng.z.yan@intel.com> wrote:
>>>> From: "Yan, Zheng" <zheng.z.yan@intel.com>
>>>>
>>>> Try enabling the LBR call stack feature if event request recording
>>>> callchain. Try utilizing the LBR call stack to get user callchain
>>>> in case of there is no frame pointer.
>>>>
>>>> Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
>>>> ---
>>>>  arch/x86/kernel/cpu/perf_event.c           | 126 +++++++++++++++++++++--------
>>>>  arch/x86/kernel/cpu/perf_event.h           |   7 ++
>>>>  arch/x86/kernel/cpu/perf_event_intel.c     |  20 ++---
>>>>  arch/x86/kernel/cpu/perf_event_intel_lbr.c |   3 +
>>>>  include/linux/perf_event.h                 |   6 ++
>>>>  kernel/events/core.c                       |  11 ++-
>>>>  6 files changed, 124 insertions(+), 49 deletions(-)
>>>>
>>>> diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
>>>> index 8ae8044..3bf2100 100644
>>>> --- a/arch/x86/kernel/cpu/perf_event.c
>>>> +++ b/arch/x86/kernel/cpu/perf_event.c
>>>> @@ -398,35 +398,46 @@ int x86_pmu_hw_config(struct perf_event *event)
>>>>
>>>>                 if (event->attr.precise_ip > precise)
>>>>                         return -EOPNOTSUPP;
>>>> -               /*
>>>> -                * check that PEBS LBR correction does not conflict with
>>>> -                * whatever the user is asking with attr->branch_sample_type
>>>> -                */
>>>> -               if (event->attr.precise_ip > 1 && x86_pmu.intel_cap.pebs_format < 2) {
>>>> -                       u64 *br_type = &event->attr.branch_sample_type;
>>>> -
>>>> -                       if (has_branch_stack(event)) {
>>>> -                               if (!precise_br_compat(event))
>>>> -                                       return -EOPNOTSUPP;
>>>> -
>>>> -                               /* branch_sample_type is compatible */
>>>> -
>>>> -                       } else {
>>>> -                               /*
>>>> -                                * user did not specify  branch_sample_type
>>>> -                                *
>>>> -                                * For PEBS fixups, we capture all
>>>> -                                * the branches at the priv level of the
>>>> -                                * event.
>>>> -                                */
>>>> -                               *br_type = PERF_SAMPLE_BRANCH_ANY;
>>>> -
>>>> -                               if (!event->attr.exclude_user)
>>>> -                                       *br_type |= PERF_SAMPLE_BRANCH_USER;
>>>> -
>>>> -                               if (!event->attr.exclude_kernel)
>>>> -                                       *br_type |= PERF_SAMPLE_BRANCH_KERNEL;
>>>> -                       }
>>>> +       }
>>>> +       /*
>>>> +        * check that PEBS LBR correction does not conflict with
>>>> +        * whatever the user is asking with attr->branch_sample_type
>>>> +        */
>>>> +       if (event->attr.precise_ip > 1 && x86_pmu.intel_cap.pebs_format < 2) {
>>>> +               u64 *br_type = &event->attr.branch_sample_type;
>>>> +
>>>> +               if (has_branch_stack(event)) {
>>>> +                       if (!precise_br_compat(event))
>>>> +                               return -EOPNOTSUPP;
>>>> +
>>>> +                       /* branch_sample_type is compatible */
>>>> +
>>>> +               } else {
>>>> +                       /*
>>>> +                        * user did not specify  branch_sample_type
>>>> +                        *
>>>> +                        * For PEBS fixups, we capture all
>>>> +                        * the branches at the priv level of the
>>>> +                        * event.
>>>> +                        */
>>>> +                       *br_type = PERF_SAMPLE_BRANCH_ANY;
>>>> +
>>>> +                       if (!event->attr.exclude_user)
>>>> +                               *br_type |= PERF_SAMPLE_BRANCH_USER;
>>>> +
>>>> +                       if (!event->attr.exclude_kernel)
>>>> +                               *br_type |= PERF_SAMPLE_BRANCH_KERNEL;
>>>> +               }
>>>> +       } else if (event->attr.sample_type & PERF_SAMPLE_CALLCHAIN) {
>>>> +               if (!has_branch_stack(event) && x86_pmu.attr_lbr_callstack) {
>>>> +                       /*
>>>> +                        * user did not specify branch_sample_type,
>>>> +                        * try using the LBR call stack facility to
>>>> +                        * record call chains in the user space.
>>>> +                        */
>>>> +                       event->attr.branch_sample_type =
>>>> +                               PERF_SAMPLE_BRANCH_USER |
>>>> +                               PERF_SAMPLE_BRANCH_CALL_STACK;
>>>
>>> You are forcing user level here, but how do you know the user wanted
>>> ONLY user level
>>> callchains?
>>>
>>>
>>
>> The LBR call stack is used only when the frame pointer approach doesn't work.
> 
> And where is that determination made?

check code that is added to perf_callchain_user and perf_callchain_user32 

> 
>> I think the kernel has frame pointer for the most cases. The second reason is
>> that the LBR call stack only has 16 entries. I think it's too small to record
>> both kernel and user call chains.
>>
> It's even too small for many object oriented user programs as well.
> 
>> Regards
>> Yan, Zheng
>>