linux-kernel - Re: [PATCH V4 07/16] perf, x86: track number of events that use LBR callstack

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Date:	Thu, 03 Jul 2014 13:59:53 +0800
From:	"Yan, Zheng" <zheng.z.yan@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	linux-kernel@...r.kernel.org, mingo@...nel.org, acme@...radead.org,
	eranian@...gle.com, andi@...stfloor.org
Subject: Re: [PATCH V4 07/16] perf, x86: track number of events that use LBR
 callstack

On 07/02/2014 06:21 PM, Peter Zijlstra wrote:
> On Mon, Jun 30, 2014 at 04:50:44PM +0800, Yan, Zheng wrote:
>> +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;
>> @@ -214,6 +220,10 @@ 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 (branch_user_callstack(cpuc->br_sel))
>> +		task_ctx->lbr_callstack_users++;
>> +
> 
> So what happens if we make a per-cpu event which satisfies
> branch_user_callstack() ?
> 

So far only per-process event can satisfy branch_user_callstack(). I will fix the potential null pointer dereference.

Regards
Yan，Zheng

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