Re: [PATCH v1 2/3] perf arm-spe: Correct sample flags for dummy event

From: James Clark <james.clark@arm.com>
To: Leo Yan <leo.yan@linaro.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	 John Garry <john.garry@huawei.com>,
	Will Deacon <will@kernel.org>,
	Mathieu Poirier <mathieu.poirier@linaro.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
	Al Grant <Al.Grant@arm.com>
Subject: Re: [PATCH v1 2/3] perf arm-spe: Correct sample flags for dummy event
Date: Wed, 12 May 2021 17:39:56 +0300	[thread overview]
Message-ID: <f4e483ae-acbb-7afa-c215-cb4244c2e820@arm.com> (raw)
In-Reply-To: <20210429150100.282180-3-leo.yan@linaro.org>

On 29/04/2021 18:00, Leo Yan wrote:
> The dummy event is mainly used for mmap, the TIME sample is only needed
> for per-cpu case so that the perf tool can rely on the correct timing
> for parsing symbols.  And the CPU sample is useless for mmap.
> 
> This patch enables TIME sample for per-cpu mmap and doesn't enable CPU
> sample.  For later extension (e.g. support multiple AUX events), it sets
> the dummy event when the condition "opts->full_auxtrace" is true.
> 
> Signed-off-by: Leo Yan <leo.yan@linaro.org>
> ---
>  tools/perf/arch/arm64/util/arm-spe.c | 30 ++++++++++++++++------------
>  1 file changed, 17 insertions(+), 13 deletions(-)
> 
> diff --git a/tools/perf/arch/arm64/util/arm-spe.c b/tools/perf/arch/arm64/util/arm-spe.c
> index 902e73a64184..f6eec0900604 100644
> --- a/tools/perf/arch/arm64/util/arm-spe.c
> +++ b/tools/perf/arch/arm64/util/arm-spe.c
> @@ -70,7 +70,6 @@ static int arm_spe_recording_options(struct auxtrace_record *itr,
>  	struct evsel *evsel, *arm_spe_evsel = NULL;
>  	struct perf_cpu_map *cpus = evlist->core.cpus;
>  	bool privileged = perf_event_paranoid_check(-1);
> -	struct evsel *tracking_evsel;
>  	int err;
>  
>  	sper->evlist = evlist;
> @@ -126,18 +125,23 @@ static int arm_spe_recording_options(struct auxtrace_record *itr,
>  		evsel__set_sample_bit(arm_spe_evsel, CPU);
>  
>  	/* Add dummy event to keep tracking */
> -	err = parse_events(evlist, "dummy:u", NULL);
> -	if (err)
> -		return err;
> -
> -	tracking_evsel = evlist__last(evlist);
> -	evlist__set_tracking_event(evlist, tracking_evsel);
> -
> -	tracking_evsel->core.attr.freq = 0;
> -	tracking_evsel->core.attr.sample_period = 1;
> -	evsel__set_sample_bit(tracking_evsel, TIME);
> -	evsel__set_sample_bit(tracking_evsel, CPU);
> -	evsel__reset_sample_bit(tracking_evsel, BRANCH_STACK);
> +	if (opts->full_auxtrace) {
> +		struct evsel *tracking_evsel;

Hi Leo,

I know the "if (opts->full_auxtrace)" pattern is copied from other auxtrace
files, but I don't think it does anything because there is this at the top
of the function:

   	if (!opts->full_auxtrace)
		return 0;

The same applies for other usages of "full_auxtrace" in the same function.
They are all always true. I'm also not sure if it's ever defined what
full_auxtrace means.

James

> +
> +		err = parse_events(evlist, "dummy:u", NULL);
> +		if (err)
> +			return err;
> +
> +		tracking_evsel = evlist__last(evlist);
> +		evlist__set_tracking_event(evlist, tracking_evsel);
> +
> +		tracking_evsel->core.attr.freq = 0;
> +		tracking_evsel->core.attr.sample_period = 1;
> +
> +		/* In per-cpu case, always need the time of mmap events etc */
> +		if (!perf_cpu_map__empty(cpus))
> +			evsel__set_sample_bit(tracking_evsel, TIME);
> +	}
>  
>  	return 0;
>  }
> 

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