linux-kernel - Re: [PATCH V5 08/16] perf, x86: track number of events that use LBR callstack

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Date:	Wed, 24 Sep 2014 14:53:49 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	kan.liang@...el.com
Cc:	eranian@...gle.com, linux-kernel@...r.kernel.org, mingo@...hat.com,
	paulus@...ba.org, acme@...nel.org, ak@...ux.intel.com,
	"Yan, Zheng" <zheng.z.yan@...el.com>
Subject: Re: [PATCH V5 08/16] perf, x86: track number of events that use LBR
 callstack

On Wed, Sep 10, 2014 at 10:09:05AM -0400, kan.liang@...el.com wrote:
> @@ -204,9 +204,15 @@ void intel_pmu_lbr_sched_task(struct perf_event_context *ctx, bool sched_in)
>  	}
>  }
>  
> +static inline bool branch_user_callstack(unsigned br_sel)
> +{
> +	return (br_sel & X86_BR_USER) && (br_sel & X86_BR_CALL_STACK);
> +}
> +
>  void intel_pmu_lbr_enable(struct perf_event *event)
>  {
>  	struct cpu_hw_events *cpuc = &__get_cpu_var(cpu_hw_events);
> +	struct x86_perf_task_context *task_ctx;
>  
>  	if (!x86_pmu.lbr_nr)
>  		return;
> @@ -220,6 +226,10 @@ void intel_pmu_lbr_enable(struct perf_event *event)
>  	}
>  	cpuc->br_sel = event->hw.branch_reg.reg;
>  
> +	task_ctx = event->ctx ? event->ctx->task_ctx_data : NULL;
> +	if (task_ctx && branch_user_callstack(cpuc->br_sel))
> +		task_ctx->lbr_callstack_users++;
> +

Does it make sense to flip those conditions to avoid a potentially
useless dereference?
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