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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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