Message-ID: <Z4EsUAtOKZUzcw2S@x1>
Date: Fri, 10 Jan 2025 11:18:56 -0300
From: Arnaldo Carvalho de Melo <acme@...nel.org>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	Ian Rogers <irogers@...gle.com>,
	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@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Kan Liang <kan.liang@...ux.intel.com>,
	James Clark <james.clark@...aro.org>, Ze Gao <zegao2021@...il.com>,
	Weilin Wang <weilin.wang@...el.com>,
	Dominique Martinet <asmadeus@...ewreck.org>,
	Jean-Philippe Romain <jean-philippe.romain@...s.st.com>,
	Junhao He <hejunhao3@...wei.com>, linux-perf-users@...r.kernel.org,
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
	Aditya Bodkhe <Aditya.Bodkhe1@....com>, Leo Yan <leo.yan@....com>,
	Atish Patra <atishp@...osinc.com>
Subject: Re: [PATCH v5 3/4] perf record: Skip don't fail for events that
 don't open

Adding Linus to the CC list as he participated in this discussion in the
past, so a heads up about changes in this area that are being further
discussed.

On Thu, Jan 09, 2025 at 05:25:03PM -0800, Namhyung Kim wrote:
> On Thu, Jan 09, 2025 at 02:21:08PM -0800, Ian Rogers wrote:
> > Whilst for many tools it is an expected behavior that failure to open
> > a perf event is a failure, ARM decided to name PMU events the same as
> > legacy events and then failed to rename such events on a server uncore
> > SLC PMU. As perf's default behavior when no PMU is specified is to
> > open the event on all PMUs that advertise/"have" the event, this
> > yielded failures when trying to make the priority of legacy and
> > sysfs/json events uniform - something requested by RISC-V and ARM. A
> > legacy event user on ARM hardware may find their event opened on an
> > uncore PMU which for perf record will fail. Arnaldo suggested skipping
> > such events which this patch implements. Rather than have the skipping
> > conditional on running on ARM, the skipping is done on all
> > architectures as such a fundamental behavioral difference could lead
> > to problems with tools built/depending on perf.
> > 
> > An example of perf record failing to open events on x86 is:
> > ```
> > $ perf record -e data_read,cycles,LLC-prefetch-read -a sleep 0.1
> > Error:
> > Failure to open event 'data_read' on PMU 'uncore_imc_free_running_0' which will be removed.
> > The sys_perf_event_open() syscall returned with 22 (Invalid argument) for event (data_read).
> > "dmesg | grep -i perf" may provide additional information.
> > 
> > Error:
> > Failure to open event 'data_read' on PMU 'uncore_imc_free_running_1' which will be removed.
> > The sys_perf_event_open() syscall returned with 22 (Invalid argument) for event (data_read).
> > "dmesg | grep -i perf" may provide additional information.
> > 
> > Error:
> > Failure to open event 'LLC-prefetch-read' on PMU 'cpu' which will be removed.
> > The LLC-prefetch-read event is not supported.
> > [ perf record: Woken up 1 times to write data ]
> > [ perf record: Captured and wrote 2.188 MB perf.data (87 samples) ]
> 
> I'm afraid this can be too noisy.

Agreed.
 
> > $ perf report --stats
> > Aggregated stats:
> >                TOTAL events:      17255
> >                 MMAP events:        284  ( 1.6%)
> >                 COMM events:       1961  (11.4%)
> >                 EXIT events:          1  ( 0.0%)
> >                 FORK events:       1960  (11.4%)
> >               SAMPLE events:         87  ( 0.5%)
> >                MMAP2 events:      12836  (74.4%)
> >              KSYMBOL events:         83  ( 0.5%)
> >            BPF_EVENT events:         36  ( 0.2%)
> >       FINISHED_ROUND events:          2  ( 0.0%)
> >             ID_INDEX events:          1  ( 0.0%)
> >           THREAD_MAP events:          1  ( 0.0%)
> >              CPU_MAP events:          1  ( 0.0%)
> >            TIME_CONV events:          1  ( 0.0%)
> >        FINISHED_INIT events:          1  ( 0.0%)
> > cycles stats:
> >               SAMPLE events:         87
> > ```
> > 
> > If all events fail to open then the perf record will fail:
> > ```
> > $ perf record -e LLC-prefetch-read true
> > Error:
> > Failure to open event 'LLC-prefetch-read' on PMU 'cpu' which will be removed.
> > The LLC-prefetch-read event is not supported.
> > Error:
> > Failure to open any events for recording
> > ```
> > 
> > As an evlist may have dummy events that open when all command line
> > events fail we ignore dummy events when detecting if at least some
> > events open. This still permits the dummy event on its own to be used
> > as a permission check:
> > ```
> > $ perf record -e dummy true
> > [ perf record: Woken up 1 times to write data ]
> > [ perf record: Captured and wrote 0.046 MB perf.data ]
> > ```
> > but allows failure when a dummy event is implicilty inserted or when
> > there are insufficient permissions to open it:
> > ```
> > $ perf record -e LLC-prefetch-read -a true
> > Error:
> > Failure to open event 'LLC-prefetch-read' on PMU 'cpu' which will be removed.
> > The LLC-prefetch-read event is not supported.
> > Error:
> > Failure to open any events for recording
> > ```
> > 
> > The issue with legacy events is that on RISC-V they want the driver to
> > not have mappings from legacy to non-legacy config encodings for each
> > vendor/model due to size, complexity and difficulty to update. It was
> > reported that on ARM Apple-M? CPUs the legacy mapping in the driver
> > was broken and the sysfs/json events should always take precedent,
> > however, it isn't clear this is still the case. It is the case that
> > without working around this issue a legacy event like cycles without a
> > PMU can encode differently than when specified with a PMU - the
> > non-PMU version favoring legacy encodings, the PMU one avoiding legacy
> > encodings.
> > 
> > The patch removes events and then adjusts the idx value for each
> > evsel. This is done so that the dense xyarrays used for file
> > descriptors, etc. don't contain broken entries. As event opening
> > happens relatively late in the record process, use of the idx value
> > before the open will have become corrupted, so it is expected there
> > are latent bugs hidden behind this change - the change is best
> > effort. As the only vendor that has broken event names is ARM, this
> > will principally effect ARM users. They will also experience warning
> > messages like those above because of the uncore PMU advertising legacy
> > event names.
> > 
> > Suggested-by: Arnaldo Carvalho de Melo <acme@...nel.org>
> > Signed-off-by: Ian Rogers <irogers@...gle.com>
> > Tested-by: James Clark <james.clark@...aro.org>
> > Tested-by: Leo Yan <leo.yan@....com>
> > Tested-by: Atish Patra <atishp@...osinc.com>
> > ---
> >  tools/perf/builtin-record.c | 47 ++++++++++++++++++++++++++++++++-----
> >  1 file changed, 41 insertions(+), 6 deletions(-)
> > 
> > diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> > index 5db1aedf48df..c0b8249a3787 100644
> > --- a/tools/perf/builtin-record.c
> > +++ b/tools/perf/builtin-record.c
> > @@ -961,7 +961,6 @@ static int record__config_tracking_events(struct record *rec)
> >  	 */
> >  	if (opts->target.initial_delay || target__has_cpu(&opts->target) ||
> >  	    perf_pmus__num_core_pmus() > 1) {
> > -
> >  		/*
> >  		 * User space tasks can migrate between CPUs, so when tracing
> >  		 * selected CPUs, sideband for all CPUs is still needed.
> > @@ -1366,6 +1365,7 @@ static int record__open(struct record *rec)
> >  	struct perf_session *session = rec->session;
> >  	struct record_opts *opts = &rec->opts;
> >  	int rc = 0;
> > +	bool skipped = false;
> >  
> >  	evlist__for_each_entry(evlist, pos) {
> >  try_again:
> > @@ -1381,15 +1381,50 @@ static int record__open(struct record *rec)
> >  			        pos = evlist__reset_weak_group(evlist, pos, true);
> >  				goto try_again;
> >  			}
> > -			rc = -errno;
> >  			evsel__open_strerror(pos, &opts->target, errno, msg, sizeof(msg));
> > -			ui__error("%s\n", msg);
> > -			goto out;
> > +			ui__error("Failure to open event '%s' on PMU '%s' which will be removed.\n%s\n",
> > +				  evsel__name(pos), evsel__pmu_name(pos), msg);
 
> How about changing it to pr_debug() and add below ...

That sounds better.
 
> > +			pos->skippable = true;
> > +			skipped = true;
> > +		} else {
> > +			pos->supported = true;
> >  		}
> > -
> > -		pos->supported = true;
> >  	}
> >  
> > +	if (skipped) {
> > +		struct evsel *tmp;
> > +		int idx = 0;
> > +		bool evlist_empty = true;
> > +
> > +		/* Remove evsels that failed to open and update indices. */
> > +		evlist__for_each_entry_safe(evlist, tmp, pos) {
> > +			if (pos->skippable) {
> > +				evlist__remove(evlist, pos);
> > +				continue;
> > +			}
> > +
> > +			/*
> > +			 * Note, dummy events may be command line parsed or
> > +			 * added by the tool. We care about supporting `perf
> > +			 * record -e dummy` which may be used as a permission
> > +			 * check. Dummy events that are added to the command
> > +			 * line and opened along with other events that fail,
> > +			 * will still fail as if the dummy events were tool
> > +			 * added events for the sake of code simplicity.
> > +			 */
> > +			if (!evsel__is_dummy_event(pos))
> > +				evlist_empty = false;
> > +		}
> > +		evlist__for_each_entry(evlist, pos) {
> > +			pos->core.idx = idx++;
> > +		}
> > +		/* If list is empty then fail. */
> > +		if (evlist_empty) {
> > +			ui__error("Failure to open any events for recording.\n");
> > +			rc = -1;
> > +			goto out;
> > +		}

> ... ?

> 		if (!verbose)
> 			ui__warning("Removed some unsupported events, use -v for details.\n");

And even this one would be best left for cases where we can determine
that its a new situation, i.e. one that should work and not the ones we
know that will not work already and thus so far didn't alarm the user
into thinking something is wrong.

Having the ones we know will fail as pr_debug() seems enough, I'd say.

- Arnaldo
 
> Thanks,
> Namhyung
> 
> 
> > +	}
> >  	if (symbol_conf.kptr_restrict && !evlist__exclude_kernel(evlist)) {
> >  		pr_warning(
> >  "WARNING: Kernel address maps (/proc/{kallsyms,modules}) are restricted,\n"
> > -- 
> > 2.47.1.613.gc27f4b7a9f-goog
> > 

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