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Message-ID: <5087D678.1080905@intel.com>
Date: Wed, 24 Oct 2012 19:52:24 +0800
From: "Yan, Zheng" <zheng.z.yan@...el.com>
To: Stephane Eranian <eranian@...gle.com>
CC: LKML <linux-kernel@...r.kernel.org>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
"ak@...ux.intel.com" <ak@...ux.intel.com>
Subject: Re: [PATCH V2 6/7] perf, x86: Use LBR call stack to get user callchain
On 10/24/2012 07:47 PM, Stephane Eranian wrote:
> On Wed, Oct 24, 2012 at 1:23 PM, Yan, Zheng <zheng.z.yan@...el.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@...el.com> wrote:
>>>> From: "Yan, Zheng" <zheng.z.yan@...el.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@...el.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
>>
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