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

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Date:   Wed, 12 May 2021 23:23:30 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     James Clark <james.clark@....com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        John Garry <john.garry@...wei.com>,
        Will Deacon <will@...nel.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Al Grant <Al.Grant@....com>
Subject: Re: [PATCH v1 2/3] perf arm-spe: Correct sample flags for dummy event

On Wed, May 12, 2021 at 05:39:56PM +0300, James Clark wrote:
> 
> 
> 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@...aro.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.

Good pointing.  TBH, I also stuggled for handling "opts->full_auxtrace"
when wrote the patch; IIUC, "opts->full_auxtrace" is also used in
builtin-record.c to indicate if the recording contains AUX tracing.

Will follow your suggestion to respin the patch (and refine the code)
to remove the redundant condition checking for "opts->full_auxtrace".

Thanks,
Leo