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Message-ID: <5087CF96.7010406@intel.com>
Date: Wed, 24 Oct 2012 19:23:02 +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 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.
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.
Regards
Yan, Zheng
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