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SubjectRe: [PATCH 04/13] perf_events: sync branch stack sampling with X86 precise_sampling (v3)
On Monday 09 January 2012 10:19 PM, Stephane Eranian wrote:
> If precise sampling is enabled on Intel X86, then perf_event uses PEBS.
> To correct for the off-by-one error of PEBS, perf_event uses LBR when
> precise_sample > 1.
>
> On Intel X86 PERF_SAMPLE_BRANCH_STACK is implemented using LBR,
> therefore both features must be coordinated as they may not
> configure LBR the same way.
>
> For PEBS, LBR needs to capture all branches at all priv levels.
> This patch sets this up.
>
> The configuration of PERF_SAMPLE_BRANCH_STACK may not be compatible
> in which case an error must be returned.
>
> Signed-off-by: Stephane Eranian <eranian@google.com>
Reviewed by: Anshuman Khandual<khandual@linux.vnet.ibm.com>
> ---
> arch/x86/kernel/cpu/perf_event.c | 22 ++++++++++++++++++++++
> 1 files changed, 22 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
> index 3779313..710ec93 100644
> --- a/arch/x86/kernel/cpu/perf_event.c
> +++ b/arch/x86/kernel/cpu/perf_event.c
> @@ -356,6 +356,7 @@ int x86_setup_perfctr(struct perf_event *event)
> int x86_pmu_hw_config(struct perf_event *event)
> {
> if (event->attr.precise_ip) {
> + u64 *br_type, br_sel;
> int precise = 0;
>
> /* Support for constant skid */
> @@ -369,6 +370,27 @@ 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) {
> +
> + br_type = &event->attr.branch_sample_type;
> +
> + if (has_branch_stack(event)) {
> + br_sel = *br_type & PERF_SAMPLE_BRANCH_ANY;
> + if (br_sel != PERF_SAMPLE_BRANCH_ANY)
> + return -EOPNOTSUPP;
> + } else {
> + /*
> + * For PEBS fixups, we capture all
> + * the branches at all priv levels
> + */
> + *br_type = PERF_SAMPLE_BRANCH_ANY
> + | PERF_SAMPLE_BRANCH_PLM_ALL;
> + }
> + }
> }
>
> /*


--
Linux Technology Centre
IBM Systems and Technology Group
Bangalore India



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