linux-kernel - [PATCH V6 14/17] perf, x86: disable FREEZE_LBRS_ON

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Date:	Sun, 19 Oct 2014 17:55:09 -0400
From:	Kan Liang <kan.liang@...el.com>
To:	a.p.zijlstra@...llo.nl, eranian@...gle.com
Cc:	linux-kernel@...r.kernel.org, mingo@...hat.com, paulus@...ba.org,
	acme@...nel.org, jolsa@...hat.com, ak@...ux.intel.com,
	Kan Liang <kan.liang@...el.com>,
	"Yan, Zheng" <zheng.z.yan@...el.com>
Subject: [PATCH V6 14/17] perf, x86: disable FREEZE_LBRS_ON_PMI when LBR operates in callstack mode

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@...el.com>
Signed-off-by: Kan Liang <kan.liang@...el.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

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