Messages in this thread Patch in this message | | | From | Andi Kleen <> | Subject | [PATCH 2/2] perf, x86: Enable PDIR precise instruction profiling on IvyBridge | Date | Wed, 13 Jun 2012 12:20:01 -0700 |
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From: Andi Kleen <ak@linux.intel.com>
Even with precise profiling Intel CPUs have a "skid". The sample triggers a few cycles later than the instruction, so in some cases there can be systematic errors where expensive instructions never show up in the profile log.
Sandy Bridge added a new PDIR instruction retired event that randomizes the sampling slightly. This corrects for systematic errors, so that you should in most cases see the correct instruction getting profile hits.
Unfortunately the SandyBridge version could only work with a otherwise quiescent CPU and was difficult to use. But now on IvyBridge this restriction is gone and can be more widely used.
This only works for retired instructions.
I enabled it -- somewhat arbitarily -- for two 'p's or more.
To use it
perf record -e instructions:pp ...
This provides a more precise alternative to the usual cycles:pp, however it will not account for expensive instructions.
Signed-off-by: Andi Kleen <ak@linux.intel.com> --- arch/x86/kernel/cpu/perf_event_intel.c | 23 +++++++++++++++++++++++ 1 files changed, 23 insertions(+), 0 deletions(-)
diff --git a/arch/x86/kernel/cpu/perf_event_intel.c b/arch/x86/kernel/cpu/perf_event_intel.c index abb29c2..886d124 100644 --- a/arch/x86/kernel/cpu/perf_event_intel.c +++ b/arch/x86/kernel/cpu/perf_event_intel.c @@ -1425,6 +1425,28 @@ static int intel_pmu_hw_config(struct perf_event *event) return 0; } +static int pdir_hw_config(struct perf_event *event) +{ + int err = intel_pmu_hw_config(event); + + if (err) + return err; + + /* + * Use the PDIR instruction retired counter for two 'p's. + * This will randomize samples slightly and avoid some systematic + * measurement errors. + * Only works for retired cycles. + */ + if (event->attr.precise_ip >= 2 && + (event->hw.config & X86_RAW_EVENT_MASK) == 0xc0) { + u64 pdir_event = X86_CONFIG(.event=0xc0, .umask=1); + event->hw.config = pdir_event | (event->hw.config & ~X86_RAW_EVENT_MASK); + } + + return 0; +} + struct perf_guest_switch_msr *perf_guest_get_msrs(int *nr) { if (x86_pmu.guest_get_msrs) @@ -1943,6 +1965,7 @@ __init int intel_pmu_init(void) x86_pmu.event_constraints = intel_snb_event_constraints; x86_pmu.pebs_constraints = intel_snb_pebs_event_constraints; x86_pmu.extra_regs = intel_snb_extra_regs; + x86_pmu.hw_config = pdir_hw_config; /* all extra regs are per-cpu when HT is on */ x86_pmu.er_flags |= ERF_HAS_RSP_1; x86_pmu.er_flags |= ERF_NO_HT_SHARING; -- 1.7.7.6
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