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Date: Wed, 24 Oct 2012 13:47:38 +0200
From: Stephane Eranian <eranian@...gle.com>
To: "Yan, Zheng" <zheng.z.yan@...el.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 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?
> 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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