linux-kernel - Re: [PATCH V4 1/3] perf tools: enable LBR call stack support

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Date:	Wed, 19 Nov 2014 16:05:43 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	kan.liang@...el.com
Cc:	acme@...nel.org, jolsa@...hat.com, a.p.zijlstra@...llo.nl,
	eranian@...gle.com, linux-kernel@...r.kernel.org, mingo@...hat.com,
	paulus@...ba.org, ak@...ux.intel.com
Subject: Re: [PATCH V4 1/3] perf tools: enable LBR call stack support

On Tue, 18 Nov 2014 16:36:55 -0500, kan liang wrote:
> From: Kan Liang <kan.liang@...el.com>
>
> Currently, there are two call chain recording options, fp and dwarf.
> Haswell has a new feature that utilizes the existing LBR facility to
> record call chains. So it provides the third options to record call
> chain. This patch enables the lbr call stack support.
>
> LBR call stack has some limitations. It reuses current LBR facility, so
> LBR call stack and branch record can not be enabled at the same time. It
> is only available for user callchain.
> However, LBR call stack can work on the user app which doesn't have
> frame-pointer or dwarf debug info compiled. It is a good alternative
> when nothing else works.


[SNIP]
>  static void
> -perf_evsel__config_callgraph(struct perf_evsel *evsel)
> +perf_evsel__config_callgraph(struct perf_evsel *evsel,
> +			     struct record_opts *opts)
>  {
>  	bool function = perf_evsel__is_function_event(evsel);
>  	struct perf_event_attr *attr = &evsel->attr;
>  
>  	perf_evsel__set_sample_bit(evsel, CALLCHAIN);
>  
> +	if (callchain_param.record_mode == CALLCHAIN_LBR) {
> +		if (!opts->branch_stack) {
> +			perf_evsel__set_sample_bit(evsel, BRANCH_STACK);
> +			attr->branch_sample_type = PERF_SAMPLE_BRANCH_USER |
> +						PERF_SAMPLE_BRANCH_CALL_STACK;
> +			if (attr->exclude_user) {
> +				attr->exclude_user = 0;
> +
> +				pr_warning("LBR callstack option is only available"
> +					   " to get user callchain information."
> +					   " Force exclude_user to 0.\n");
> +			}

I'm not sure what's in a higher priority - maybe I missed earlier
discussion.  IOW what about this?

			if (attr->exclude_user) {
				pr_warning("LBR callstack option is only available"
					   " to get user callchain information.\n");
			} else {
				perf_evsel__set_sample_bit(evsel, BRANCH_STACK);
				attr->branch_sample_type = PERF_SAMPLE_BRANCH_USER |
							PERF_SAMPLE_BRANCH_CALL_STACK;
			}

> +		} else
> +			 pr_info("Cannot use LBR callstack with branch stack\n");

It seems pr_warning is more appropriate here.

Thanks,
Namhyung


> +	}
> +
>  	if (callchain_param.record_mode == CALLCHAIN_DWARF) {
>  		if (!function) {
>  			perf_evsel__set_sample_bit(evsel, REGS_USER);
> @@ -659,7 +676,7 @@ void perf_evsel__config(struct perf_evsel *evsel, struct record_opts *opts)
>  	}
>  
>  	if (callchain_param.enabled && !evsel->no_aux_samples)
> -		perf_evsel__config_callgraph(evsel);
> +		perf_evsel__config_callgraph(evsel, opts);
>  
>  	if (target__has_cpu(&opts->target))
>  		perf_evsel__set_sample_bit(evsel, CPU);
--
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