[PATCH V6 14/17] perf, x86: disable FREEZE_LBRS_ON_PMI when LBR operates in callstack mode

Kan Liang
2014-10-19T22:07:43Z

LBR callstack is designed for PEBS, It does not work well with
FREEZE_LBRS_ON_PMI for non PEBS event. If FREEZE_LBRS_ON_PMI is set for
non PEBS event, PMIs near call/return instructions may cause superfluous
increase/decrease of LBR_TOS.

This patch modifies __intel_pmu_lbr_enable() to not enable
FREEZE_LBRS_ON_PMI when LBR operates in callstack mode. We currently
don't use LBR callstack to capture kernel space callchain, so disabling
FREEZE_LBRS_ON_PMI should not be a problem.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Signed-off-by: Kan Liang <kan.liang@intel.com>
---
arch/x86/kernel/cpu/perf_event_intel_lbr.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event_intel_lbr.c b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
index 6e473c9..cd4bd6f 100644
--- a/arch/x86/kernel/cpu/perf_event_intel_lbr.c
+++ b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
@@ -131,14 +131,23 @@ static void intel_pmu_lbr_filter(struct cpu_hw_events *cpuc);

static void __intel_pmu_lbr_enable(void)
{
- u64 debugctl;
struct cpu_hw_events *cpuc = &__get_cpu_var(cpu_hw_events);
+ u64 debugctl, lbr_select = 0;

- if (cpuc->lbr_sel)
- wrmsrl(MSR_LBR_SELECT, cpuc->lbr_sel->config);
+ if (cpuc->lbr_sel) {
+ lbr_select = cpuc->lbr_sel->config;
+ wrmsrl(MSR_LBR_SELECT, lbr_select);
+ }

rdmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);
- debugctl |= (DEBUGCTLMSR_LBR | DEBUGCTLMSR_FREEZE_LBRS_ON_PMI);
+ debugctl |= DEBUGCTLMSR_LBR;
+ /*
+ * LBR callstack does not work well with FREEZE_LBRS_ON_PMI.
+ * If FREEZE_LBRS_ON_PMI is set, PMI near call/return instructions
+ * may cause superfluous increase/decrease of LBR_TOS.
+ */
+ if (!(lbr_select & LBR_CALL_STACK))
+ debugctl |= DEBUGCTLMSR_FREEZE_LBRS_ON_PMI;
wrmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);
}

--
1.8.3.2