Re: [PATCH V5 09/16] perf, x86: Save/resotre LBR stack during context switch

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Sep 10, 2014 at 10:09:06AM -0400, kan.liang@intel.com wrote:
>  void intel_pmu_lbr_sched_task(struct perf_event_context *ctx, bool sched_in)
>  {
>  	struct cpu_hw_events *cpuc = &__get_cpu_var(cpu_hw_events);
> +	struct x86_perf_task_context *task_ctx;
>  
>  	if (!x86_pmu.lbr_nr)
>  		return;
>  	/*
> +	 * If LBR callstack feature is enabled and the stack was saved when
> +	 * the task was scheduled out, restore the stack. Otherwise flush
> +	 * the LBR stack.
> +	 */
> +	task_ctx = ctx ? ctx->task_ctx_data : NULL;
> +	if (task_ctx) {
> +		if (sched_in) {
> +			__intel_pmu_lbr_restore(task_ctx);
> +			cpuc->lbr_context = ctx;
> +		} else {
> +			__intel_pmu_lbr_save(task_ctx);
> +		}
> +	}
> +
> +	/*
>  	 * When sampling the branck stack in system-wide, it may be
>  	 * necessary to flush the stack on context switch. This happens
>  	 * when the branch stack does not tag its entries with the pid

Why would we still need to reset if we did a save/restore on the branch
stack?