linux-kernel - Re: [PATCH 3/5] perf, tools: Disable branch flags/cycles for --callgraph lbr

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Date:	Wed, 21 Oct 2015 15:27:42 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	acme@...nel.org, jolsa@...nel.org, linux-kernel@...r.kernel.org,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 3/5] perf, tools: Disable branch flags/cycles for
 --callgraph lbr

On Tue, Oct 20, 2015 at 11:46:35AM -0700, Andi Kleen wrote:
> From: Andi Kleen <ak@...ux.intel.com>
> 
> Automatically disable collecting branch flags and cycles with
> --call-graph lbr. This allows avoiding a bunch of extra MSR
> reads in the PMI on Skylake.
> 
> When the kernel doesn't support the new flags they are automatically
> cleared in the fallback code.
> 
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>
> ---

Arnaldo, ACK?

>  tools/perf/util/evsel.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index 8be867c..e8724b4 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -36,6 +36,7 @@ static struct {
>  	bool cloexec;
>  	bool clockid;
>  	bool clockid_wrong;
> +	bool lbr_flags;
>  } perf_missing_features;
>  
>  static clockid_t clockid;
> @@ -573,7 +574,9 @@ perf_evsel__config_callgraph(struct perf_evsel *evsel,
>  			} else {
>  				perf_evsel__set_sample_bit(evsel, BRANCH_STACK);
>  				attr->branch_sample_type = PERF_SAMPLE_BRANCH_USER |
> -							PERF_SAMPLE_BRANCH_CALL_STACK;
> +							PERF_SAMPLE_BRANCH_CALL_STACK |
> +							PERF_SAMPLE_BRANCH_NO_CYCLES |
> +							PERF_SAMPLE_BRANCH_NO_FLAGS;
>  			}
>  		} else
>  			 pr_warning("Cannot use LBR callstack with branch stack. "
> @@ -1312,6 +1315,9 @@ fallback_missing_features:
>  		evsel->attr.mmap2 = 0;
>  	if (perf_missing_features.exclude_guest)
>  		evsel->attr.exclude_guest = evsel->attr.exclude_host = 0;
> +	if (perf_missing_features.lbr_flags)
> +		evsel->attr.branch_sample_type &= ~(PERF_SAMPLE_BRANCH_NO_FLAGS |
> +				     PERF_SAMPLE_BRANCH_NO_CYCLES);
>  retry_sample_id:
>  	if (perf_missing_features.sample_id_all)
>  		evsel->attr.sample_id_all = 0;
> @@ -1414,6 +1420,12 @@ try_fallback:
>  	} else if (!perf_missing_features.sample_id_all) {
>  		perf_missing_features.sample_id_all = true;
>  		goto retry_sample_id;
> +	} else if (!perf_missing_features.lbr_flags &&
> +			(evsel->attr.branch_sample_type &
> +			 (PERF_SAMPLE_BRANCH_NO_CYCLES |
> +			  PERF_SAMPLE_BRANCH_NO_FLAGS))) {
> +		perf_missing_features.lbr_flags = true;
> +		goto fallback_missing_features;
>  	}
>  
>  out_close:
> -- 
> 2.4.3
> 
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