Re: [RFC PATCH v4 1/1] perf cs-etm: Split Coresight decode by aux records

[Mathieu Poirier]

On Tue, Jun 08, 2021 at 05:28:08PM +0300, James Clark wrote:
> 
> 
> On 07/06/2021 23:03, Mathieu Poirier wrote:
> > Hi James, 
> > 
> > Leo has beat me to the punch on most of the things I noticed in this set, so
> > this is just to complement what has already been pointed out.
> > 
> > On Fri, Jun 04, 2021 at 01:21:17PM +0800, Leo Yan wrote:
> >> Hi James,
> >>
> >> On Tue, Jun 01, 2021 at 04:07:51PM +0300, James Clark wrote:
> >>> Populate the auxtrace queues using AUX records rather than whole
> >>> auxtrace buffers.
> >>>
> >>> This is similar to the auxtrace_queues__process_index() ->
> >>> auxtrace_queues__add_indexed_event() flow where
> >>> perf_session__peek_event() is used to read AUXTRACE events out of random
> >>> positions in the file based on the auxtrace index.
> >>>
> >>> Now there is a second pass using perf_session__peek_events() which loops
> >>> over all PERF_RECORD_AUX events, finds the corresponding auxtrace
> >>> buffer, and adds a fragment of that buffer to the auxtrace queues,
> >>> rather than the whole buffer.
> > 
> > The first loop is enacted by perf_session__peek_events() and processes all the
> > PERF_RECORD_AUX events.  The second loop correlates PERF_RECORD_AUX with
> > PERF_RECORD_AUXTRACE.
> > 
> > The above paragraph seems to indicate the opposite.
> > 
> 
> Hi Mathieu,
> 
> Yes, you're right I will reword this. The rest of the comments I also agree
> with so I will fix them up and submit v5.
> 
> >>>
> >>> This is a completely new implementation compared to v1 and v2 of the
> >>> patchset where significant modifications were made to the decoding flow,
> >>> including saving all of the AUX records, and changing the point where
> >>> decoding started. These changes had effects on the outcome of decoding,
> >>> but with this implementation, the output of decoding is identical to
> >>> perf/core, except in the case where decoding failed due to the issue
> >>> that this change addresses. No changes to decoding were made, apart
> >>> from populating the auxtrace queues.
> >>
> >> IMHO, this patch is much better than previous versions! :)
> >>
> >> When went through the whole patch, the most code actually is general
> >> enough and can consider to place into into util/auxtrace.c.  If there
> >> have any objection, we can come back to add the code for only cs-etm
> >> specific.
> > 
> > I would leave the idea of moving this to util/auxtrace.c alone for the time
> > being.
> > 
> >>
> >>> Because the decoder is already reset between each auxtrace buffer,
> >>> populating the queues with fragments of whole buffers based on AUX
> >>> records has the same affect as resetting the decoder on the AUX record
> >>> bounds during decode. But this approach only requires a change to one
> >>> part of the whole flow, rather than many parts.
> >>>
> >>> There is still one TODO about how buffers are found. Currently there is
> >>> a plain loop as a search, but this could be improved with a binary
> >>> search or keeping a record of which buffers have already been added and
> >>> starting the search from the last used place. AUX records and buffers
> >>> will come in similar orders in the file so the latter would be simpler
> >>> and probably be good enough. But I'm looking for feedback about the
> >>> approach as a whole before ironing out all of the details.
> > 
> > I think for the time being what you have here will work just fine.
> > 
> >>>
> >>> Other issues that were fixed by the previous patchsets such as fixing
> >>> the case where the first aux record comes before the first MMAP record
> >>> can now be fixed as completely independent issues.
> >>>
> > 
> > I assume you did not expect this patch to be merged and as such wrote as much as
> > you could, which is fine.  The next revision should have a changelog that will
> > concentrate on why this patch is needed.  I think the testing section (below) is
> > useful and should be kept around.
> 
> Do you think something like this is enough for the reasoning:
> 
>     The reason that this change is needed is because AUX records are emitted
>     any time tracing is disabled, for example when the process is scheduled out.
>     Because ETM was disabled and enabled again, the decoder also needs to
>     be reset to force the search for a sync packet. Otherwise there would
>     be fatal decoding errors.

Yes, that will work.  It describes _why_ this patch is needed.

> 
> Thanks
> James
> 
> > 
> >>> Testing
> >>> =======
> >>>
> >>> Testing was done with the following script, to diff the decoding results
> >>> between the patched and un-patched versions of perf:
> >>>
> >>> 	#!/bin/bash
> >>> 	set -ex
> >>>
> >>> 	$1 script -i $3 $4 > split.script
> >>> 	$2 script -i $3 $4 > default.script
> >>>
> >>> 	diff split.script default.script | head -n 20
> >>>
> >>> And it was run like this, with various itrace options depending on the
> >>> quantity of synthesised events:
> >>>
> >>> 	compare.sh ./perf-patched ./perf-default perf-per-cpu-2-threads.data --itrace=i100000ns
> >>>
> >>> No changes in output were observed in the following scenarios:
> >>>
> >>> * Simple per-cpu
> >>> 	perf record -e cs_etm/@tmc_etr0/u top
> >>>
> >>> * Per-thread, single thread
> >>> 	perf record -e cs_etm/@tmc_etr0/u --per-thread ./threads_C
> >>>
> >>> * Per-thread multiple threads (but only one thread collected data):
> >>> 	perf record -e cs_etm/@tmc_etr0/u --per-thread --pid 4596,4597
> >>>
> >>> * Per-thread multiple threads (both threads collected data):
> >>> 	perf record -e cs_etm/@tmc_etr0/u --per-thread --pid 4596,4597
> >>>
> >>> * Per-cpu explicit threads:
> >>> 	perf record -e cs_etm/@tmc_etr0/u --pid 853,854
> >>>
> >>> * System-wide (per-cpu):
> >>>     perf record -e cs_etm/@tmc_etr0/u -a
> >>>
> >>> * No data collected (no aux buffers)
> >>> 	Can happen with any command when run for a short period
> >>>
> >>> * Containing truncated records
> >>> 	Can happen with any command
> >>>
> >>> * Containing aux records with 0 size
> >>> 	Can happen with any command
> >>>
> >>> * Snapshot mode
> >>> 	perf record -e cs_etm/@tmc_etr0/u -a --snapshot
> >>>
> >>> Signed-off-by: James Clark <james.clark@xxxxxxx>
> >>> ---
> >>>  tools/perf/util/cs-etm.c | 159 ++++++++++++++++++++++++++++++++++++++-
> >>>  1 file changed, 156 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
> >>> index 64536a6ed10a..053aad4b28cf 100644
> >>> --- a/tools/perf/util/cs-etm.c
> >>> +++ b/tools/perf/util/cs-etm.c
> >>> @@ -2679,6 +2679,161 @@ static u64 *cs_etm__create_meta_blk(u64 *buff_in, int *buff_in_offset,
> >>>  	return metadata;
> >>>  }
> >>>  
> >>> +/**
> >>> + * Puts a fragment of an auxtrace buffer into the auxtrace queues based
> >>> + * on the bounds of aux_event, if it matches with the buffer that's at
> >>> + * file_offset.
> >>> + *
> >>> + * Normally, whole auxtrace buffers would be added to the queue. But we
> >>> + * want to reset the decoder for every PERF_RECORD_AUX event, and the decoder
> >>> + * is reset across each buffer, so splitting the buffers up in advance has
> >>> + * the same effect.
> >>> + */
> >>> +static int cs_etm__queue_aux_fragment(struct perf_session *session,
> >>> +						off_t file_offset, size_t sz,
> >>> +						struct perf_record_aux *aux_event,
> >>> +						struct perf_sample *sample)
> > 
> > The indentation doesn't work.  The 'o' of off_t should be aligned with the 's'
> > of struct on the first line.  
> > 
> >>> +{
> >>> +	int err;
> >>> +	char buf[PERF_SAMPLE_MAX_SIZE];
> >>> +	union perf_event *auxtrace_event_union;
> >>> +	struct perf_record_auxtrace *auxtrace_event;
> >>> +	union perf_event auxtrace_fragment;
> >>> +	bool matchesCpuPid;
> >>
> >> It's better to avoid using camel naming.
> >>
> > 
> > Leo is correct - I'm surprised checkpatch hasn't complained bitterly.
> > 
> >>> +	__u64 aux_offset;
> >>> +	struct cs_etm_auxtrace *etm = container_of(session->auxtrace,
> >>> +						   struct cs_etm_auxtrace,
> >>> +						   auxtrace);
> >>> +
> >>> +	/*
> >>> +	 * There should be a PERF_RECORD_AUXTRACE event at the file_offset that we got
> >>> +	 * from looping through the auxtrace index.
> >>> +	 */
> >>> +	err = perf_session__peek_event(session, file_offset, buf,
> >>> +				       PERF_SAMPLE_MAX_SIZE, &auxtrace_event_union, NULL);
> >>> +	if (err)
> >>> +		return err;
> >>> +	auxtrace_event = &auxtrace_event_union->auxtrace;
> >>> +	if (auxtrace_event->header.type != PERF_RECORD_AUXTRACE)
> >>> +		return -EINVAL;
> >>> +
> >>> +	if (auxtrace_event->header.size < sizeof(struct perf_record_auxtrace) ||
> >>> +		auxtrace_event->header.size != sz) {
> >>> +		return -EINVAL;
> >>> +	}
> >>> +
> >>> +	/*
> >>> +	 * In per-thread mode, CPU is set to -1, but TID will be set instead.
> >>> +	 * See auxtrace_mmap_params__set_idx()
> >>> +	 */
> >>> +	if (auxtrace_event->cpu == (__u32) -1)
> >>> +		matchesCpuPid = auxtrace_event->tid == sample->tid;
> >>> +	else
> >>> +		matchesCpuPid = auxtrace_event->cpu == sample->cpu;
> >>
> >> If "matchesCpuPid" is false, can directly bail out at here, so can
> >> avoid the below unnecessary calculations.
> >>
> >>> +
> >>> +	/*
> >>> +	 * In snapshot/overwrite mode, the head points to the end of the buffer so aux_offset needs
> >>> +	 * to have the size subtracted so it points to the beginning as in normal mode.
> >>> +	 */
> >>> +	if (aux_event->flags & PERF_AUX_FLAG_OVERWRITE)
> >>> +		aux_offset = aux_event->aux_offset - aux_event->aux_size;
> >>> +	else
> >>> +		aux_offset = aux_event->aux_offset;
> >>> +
> >>> +	if (matchesCpuPid &&
> >>> +	    aux_offset >= auxtrace_event->offset &&
> >>> +	    aux_offset + aux_event->aux_size <= auxtrace_event->offset + auxtrace_event->size) {
> >>
> >> It's possible that an event PERF_RECORD_AUXTRACE saves AUX trace data
> >> for multiple events PERF_RECORD_AUX (if we consider watermark).
> >>
> >> So here the logic makes sense for me for checking the buffer range and
> >> I don't find any issue.
> >>
> >>> +		/*
> >>> +		 * If this AUX event was inside this buffer somewhere, create a new auxtrace event
> >>> +		 * based on the sizes of the aux event, and queue that fragment.
> >>> +		 */
> >>> +		auxtrace_fragment.auxtrace = *auxtrace_event;
> >>> +		auxtrace_fragment.auxtrace.size = aux_event->aux_size;
> >>> +		auxtrace_fragment.auxtrace.offset = aux_offset;
> >>> +		file_offset += aux_offset - auxtrace_event->offset + auxtrace_event->header.size;
> >>> +		return auxtrace_queues__add_event(&etm->queues,
> >>> +					       session,
> >>> +					       &auxtrace_fragment,
> >>> +					       file_offset,
> >>> +					       NULL);
> >>> +	}
> >>> +
> >>> +	/* Wasn't inside this buffer, but there were no parse errors. 1 == 'not found' */
> >>> +	return 1;
> >>> +}
> >>> +
> >>> +static int cs_etm__queue_aux_records_cb(struct perf_session *session,
> >>> +			     union perf_event *event, u64 offset __maybe_unused,
> >>> +			     void *data __maybe_unused)
> > 
> > Same comment about the indentation.
> > 
> >>> +{
> >>> +	struct perf_sample sample;
> >>> +	int ret;
> >>> +	struct auxtrace_index_entry *ent;
> >>> +	struct auxtrace_index *auxtrace_index;
> >>> +	struct evsel *evsel;
> >>> +	size_t i;
> >>> +	struct cs_etm_auxtrace *etm = container_of(session->auxtrace,
> >>> +						   struct cs_etm_auxtrace,
> >>> +						   auxtrace);
> >>> +
> > 	struct cs_etm_auxtrace *etm = container_of(session->auxtrace,
> > 						   struct cs_etm_auxtrace,
> > 						   auxtrace);
> > 	struct auxtrace_index *auxtrace_index;
> > 	struct auxtrace_index_entry *ent;
> > 	struct perf_sample sample;
> > 	struct evsel *evsel;
> > 	size_t i;
> > 	int ret;
> > 
> >>> +	/* Don't care about any other events, we're only queuing buffers for AUX events */
> >>> +	if (event->header.type != PERF_RECORD_AUX)
> >>> +		return 0;
> >>> +
> >>> +	if (event->header.size < sizeof(struct perf_record_aux))
> >>> +		return -EINVAL;
> >>> +
> >>> +	/* Truncated Aux records can have 0 size and shouldn't result in anything being queued. */
> >>> +	if (!event->aux.aux_size)
> >>> +		return 0;
> >>> +
> >>> +	/*
> >>> +	 * Parse the sample, we need the sample_id_all data that comes after the event so that the
> >>> +	 * CPU or PID can be matched to an AUXTRACE buffer's CPU or PID.
> >>> +	 */
> >>> +	evsel = evlist__event2evsel(session->evlist, event);
> >>> +	evsel__parse_sample(evsel, event, &sample);
> >>
> >> It's good to check the return errors from evsel__parse_sample().
> >>
> >>> +
> >>> +	/*
> >>> +	 * Loop throuch the auxtrace index to find the buffer that matches up with this
> >>> +	 * aux event.
> >>> +	 * TODO: binary search?
> >>
> >> For binary search, you might need to reorg the auxtrace_index and
> >> its entries, I personally think this is not the priority.
> >>
> > 
> > Here too I agree with Leo.  
> > 
> >>> +	 */
> >>> +	list_for_each_entry(auxtrace_index, &session->auxtrace_index, list) {
> >>> +		for (i = 0; i < auxtrace_index->nr; i++) {
> >>> +			ent = &auxtrace_index->entries[i];
> >>> +			ret = cs_etm__queue_aux_fragment(session, ent->file_offset,
> >>> +							 ent->sz, &event->aux, &sample);
> >>> +			if (!ret) {
> >>> +				etm->data_queued = true;
> >>> +				return 0;
> >>> +			} else if (ret < 0) {
> >>> +				/*
> >>> +				 * Anything other than 1 is an error. Positive values are 'not
> >>> +				 * found' we just want to go onto the next one in that case.
> >>> +				 */
> >>> +				return ret;
> >>> +			}
> >>> +		}
> >>> +	}
> >>
> >> It's good to add a warning at here?  If cannot find a matched
> >> AUXRECORD event and fails to queue the fragment, it's likely caused by
> >> the AUX ring buffer's overrun.
> >>
> >>> +
> >>> +	/*
> >>> +	 * We would get here if there are no entries in the index (either no auxtrace buffers or
> >>> +	 * no index). Fail silently as there is the possibility of queueing them in
> >>> +	 * cs_etm__process_auxtrace_event() if etm->data_queued is still false;
> >>> +	 *
> >>> +	 * In that scenario, buffers will not be split by AUX records.
> >>> +	 */
> >>> +	return 0;
> >>> +}
> >>> +
> >>> +static int cs_etm__queue_aux_records(struct perf_session *session)
> >>> +{
> >>> +	return perf_session__peek_events(session, session->header.data_offset,
> >>> +					 session->header.data_size,
> >>> +					 cs_etm__queue_aux_records_cb, NULL);
> > 
> > Perfect indentation. Here cs_etm__queue_aux_records() doesn't give us much.  I
> > would simply call perf_session__peek_events() from
> > cs_etm__process_auxtrace_info().
> > 
> >>> +}
> >>> +
> >>>  int cs_etm__process_auxtrace_info(union perf_event *event,
> >>>  				  struct perf_session *session)
> >>>  {
> >>> @@ -2879,12 +3034,10 @@ int cs_etm__process_auxtrace_info(union perf_event *event,
> >>>  	if (err)
> >>>  		goto err_delete_thread;
> >>>  
> >>> -	err = auxtrace_queues__process_index(&etm->queues, session);
> >>> +	err = cs_etm__queue_aux_records(session);
> >>>  	if (err)
> >>>  		goto err_delete_thread;
> >>>  
> >>> -	etm->data_queued = etm->queues.populated;
> >>
> >> Seems to me it's no reason to remove this sentence.
> >>
> >> "etm->queues.populated" will be set when call
> >> auxtrace_queues__add_event(), so we still can assign it to
> >> "etm->data_queued" at here.
> >>
> >> Thanks,
> >> Leo
> >>
> > 
> > I'm happy with what is done in this patch.
> > 
> > Thanks,
> > Mathieu
> > 
> >>> -
> >>>  	return 0;
> >>>  
> >>>  err_delete_thread:
> >>> -- 
> >>> 2.28.0
> >>>