linux-kernel - Re: [PATCH V6 13/17] perf, x86: enable LBR callstack when recording callchain

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Date:	Fri, 24 Oct 2014 15:39:36 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Kan Liang <kan.liang@...el.com>
Cc:	a.p.zijlstra@...llo.nl, 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 V6 13/17] perf, x86: enable LBR callstack when recording
 callchain

On Sun, Oct 19, 2014 at 05:55:08PM -0400, Kan Liang wrote:
> Only enable LBR callstack when user requires fp callgraph. The feature
> is not available when PERF_SAMPLE_BRANCH_STACK or PERF_SAMPLE_STACK_USER
> is required.
> Also, this feature only affects how to get user callchain. The kernel
> callchain is always got by frame pointers.
> 
> 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.c | 18 ++++++++++++++++--
>  1 file changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
> index 9656b9e..b3256a3 100644
> --- a/arch/x86/kernel/cpu/perf_event.c
> +++ b/arch/x86/kernel/cpu/perf_event.c
> @@ -424,10 +424,24 @@ int x86_pmu_hw_config(struct perf_event *event)
>  			if (!event->attr.exclude_kernel)
>  				*br_type |= PERF_SAMPLE_BRANCH_KERNEL;
>  		}
> -	}
> +	} else if (x86_pmu_has_lbr_callstack() &&
> +		   (event->attr.sample_type & PERF_SAMPLE_CALLCHAIN) &&
> +		   !(event->attr.sample_type & PERF_SAMPLE_STACK_USER) &&
> +		   !has_branch_stack(event) &&
> +		   !event->attr.exclude_user &&
> +		   (event->attach_state & PERF_ATTACH_TASK)) {
> +		/*
> +		 * user did not specify branch_sample_type,
> +		 * try using the LBR call stack facility to
> +		 * record call chains of user program.
> +		 */
> +		event->attr.branch_sample_type =
> +			PERF_SAMPLE_BRANCH_USER |
> +			PERF_SAMPLE_BRANCH_CALL_STACK;
>  

I dont see PERF_SAMPLE_BRANCH_CALL_STACK being defind in uapi.. any reason
why I cant enable this feature explicitly?

thanks,
jirka
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