Re: [PATCH V4 15/16] perf, x86: disable FREEZE_LBRS_ON_PMI when LBR operates in callstack mode

[Peter Zijlstra <peterz@infradead.org>]

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On Mon, Jun 30, 2014 at 04:50:52PM +0800, Yan, Zheng wrote:
> 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>
> ---
>  arch/x86/kernel/cpu/perf_event_intel_lbr.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/perf_event_intel_lbr.c b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> index f059b98..dd14f67 100644
> --- a/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> @@ -138,7 +138,14 @@ static void __intel_pmu_lbr_enable(void)
>  		wrmsrl(MSR_LBR_SELECT, cpuc->lbr_sel->config);
>  
>  	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 (!cpuc->lbr_sel || !(cpuc->lbr_sel->config & LBR_CALL_STACK))
> +		debugctl |= DEBUGCTLMSR_FREEZE_LBRS_ON_PMI;
>  	wrmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);

Something like so perhaps? Its slightly bigger, but less messy.

diff --git a/arch/x86/kernel/cpu/perf_event_intel_lbr.c b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
index 9dd2459a4c73..a55670529b9e 100644
--- a/arch/x86/kernel/cpu/perf_event_intel_lbr.c
+++ b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
@@ -132,14 +132,18 @@ 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_config);
+	}
 
 	rdmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);
-	debugctl |= (DEBUGCTLMSR_LBR | DEBUGCTLMSR_FREEZE_LBRS_ON_PMI);
+	debugctl |= DEBUGCTLMSR_LBR;
+	if (!(lbr_select & LBR_CALL_STACK))
+		debugctl |= DEBUGCTLMSR_FREEZE_LBRS_ON_PMI;
 	wrmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);
 }
 

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