Re: [PATCH 3/3] perf arm-spe: Support hardware-based PID tracing

[German Gomez]

Hi Leo,

Thanks for the review.

On 06/11/2021 14:57, Leo Yan wrote:
> Hi German,
>
> On Tue, Nov 02, 2021 at 06:07:39PM +0000, German Gomez wrote:
>> If Arm SPE traces contain CONTEXT packets with PID info, use these
>> values for tracking pid of samples. Otherwise fall back to using context
>> switch events and display a message warning the user of possible timing
>> inaccuracies [1].
>>
>> [1] https://lore.kernel.org/lkml/f877cfa6-9b25-6445-3806-ca44a4042eaf@xxxxxxx/
> First of all, I'd like to clarify one thing:
>
> The pid/tid has been supported for 'per-thread' mode, the function
> arm_spe_process_timeless_queues() invokes arm_spe_set_pid_tid_cpu() to
> initialize 'speq->tid' and assign auxtrace_queue's tid to it.

Ack.

I forgot to validate per-thread mode and didn't realize it was using the
timeless flow.

>
> Thus, in this patch set we only need to consider support context packet
> for CPU wide tracing and system wide tracing.  The following comments
> are heavily based on this assumption.
>
>> Signed-off-by: German Gomez <german.gomez@xxxxxxx>
>> ---
>>  tools/perf/util/arm-spe.c | 123 ++++++++++++++++++++++++++++----------
>>  1 file changed, 92 insertions(+), 31 deletions(-)
>>
>> diff --git a/tools/perf/util/arm-spe.c b/tools/perf/util/arm-spe.c
>> index 230bc7ab2..00a409469 100644
>> --- a/tools/perf/util/arm-spe.c
>> +++ b/tools/perf/util/arm-spe.c
>> @@ -71,6 +71,7 @@ struct arm_spe {
>>  	u64				kernel_start;
>>  
>>  	unsigned long			num_events;
>> +	u8				use_ctx_pkt_for_pid;
>>  };
>>  
>>  struct arm_spe_queue {
>> @@ -226,6 +227,44 @@ static inline u8 arm_spe_cpumode(struct arm_spe *spe, u64 ip)
>>  		PERF_RECORD_MISC_USER;
>>  }
>>  
>> +static void arm_spe_set_pid_tid_cpu(struct arm_spe *spe,
>> +				    struct auxtrace_queue *queue)
>> +{
>> +	struct arm_spe_queue *speq = queue->priv;
>> +	pid_t tid;
>> +
>> +	tid = machine__get_current_tid(spe->machine, speq->cpu);
>> +	if (tid != -1) {
>> +		speq->tid = tid;
>> +		thread__zput(speq->thread);
>> +	} else
>> +		speq->tid = queue->tid;
>> +
>> +	if ((!speq->thread) && (speq->tid != -1)) {
>> +		speq->thread = machine__find_thread(spe->machine, -1,
>> +						    speq->tid);
>> +	}
>> +
>> +	if (speq->thread) {
>> +		speq->pid = speq->thread->pid_;
>> +		if (queue->cpu == -1)
>> +			speq->cpu = speq->thread->cpu;
>> +	}
>> +}
>> +
>> +static int arm_spe_set_tid(struct arm_spe_queue *speq, pid_t tid)
>> +{
>> +	struct arm_spe *spe = speq->spe;
>> +	int err = machine__set_current_tid(spe->machine, speq->cpu, tid, tid);
>> +
>> +	if (err)
>> +		return err;
>> +
>> +	arm_spe_set_pid_tid_cpu(spe, &spe->queues.queue_array[speq->queue_nr]);
>> +
>> +	return 0;
>> +}
>> +
>>  static void arm_spe_prep_sample(struct arm_spe *spe,
>>  				struct arm_spe_queue *speq,
>>  				union perf_event *event,
>> @@ -460,6 +499,13 @@ static int arm_spe_run_decoder(struct arm_spe_queue *speq, u64 *timestamp)
>>  		 * can correlate samples between Arm SPE trace data and other
>>  		 * perf events with correct time ordering.
>>  		 */
>> +
>> +		if (spe->use_ctx_pkt_for_pid) {
>> +			ret = arm_spe_set_tid(speq, speq->decoder->record.context_id);
>> +			if (ret)
>> +				return ret;
>> +		}
>> +
> If trace contains context packet, we can give it the priority.  So at
> here we can always update tid based on the latest context_id from the
> context packet.
>
> And for 'per thread' mode, we should not set pid/tid for it.  The
> reason is arm_spe_set_tid() will return error for 'per thread' mode,
> because the 'speq->cpu' is -1, so it cannot set tid/pid for CPU '-1'.
>
> And if we detect there have context packet is incoming, we should set
> the flag 'spe->use_ctx_pkt_for_pid' true.
>
> How about below code:
>
>         /* Update pid/tid info */
>         record = &speq->decoder->record;
>         if (!spe->timeless_decoding && record->context_id != (u64)-1) {
>                 ret = arm_spe_set_tid(speq, record->context_id);
>                 if (ret)
>                         return ret;
>
>                 spe->use_ctx_pkt_for_pid = 1;
>         }

Yeah it looks good. The patch defined the two method as strictly
mutually exclusive, but I prefer this way better which is also more in
line with the previous RFC. Thanks for the suggestion.

>>  		ret = arm_spe_sample(speq);
>>  		if (ret)
>>  			return ret;
>> @@ -586,31 +632,6 @@ static bool arm_spe__is_timeless_decoding(struct arm_spe *spe)
>>  	return timeless_decoding;
>>  }
>>  
>> -static void arm_spe_set_pid_tid_cpu(struct arm_spe *spe,
>> -				    struct auxtrace_queue *queue)
>> -{
>> -	struct arm_spe_queue *speq = queue->priv;
>> -	pid_t tid;
>> -
>> -	tid = machine__get_current_tid(spe->machine, speq->cpu);
>> -	if (tid != -1) {
>> -		speq->tid = tid;
>> -		thread__zput(speq->thread);
>> -	} else
>> -		speq->tid = queue->tid;
>> -
>> -	if ((!speq->thread) && (speq->tid != -1)) {
>> -		speq->thread = machine__find_thread(spe->machine, -1,
>> -						    speq->tid);
>> -	}
>> -
>> -	if (speq->thread) {
>> -		speq->pid = speq->thread->pid_;
>> -		if (queue->cpu == -1)
>> -			speq->cpu = speq->thread->cpu;
>> -	}
>> -}
>> -
>>  static int arm_spe_process_queues(struct arm_spe *spe, u64 timestamp)
>>  {
>>  	unsigned int queue_nr;
>> @@ -641,7 +662,13 @@ static int arm_spe_process_queues(struct arm_spe *spe, u64 timestamp)
>>  			ts = timestamp;
>>  		}
>>  
>> -		arm_spe_set_pid_tid_cpu(spe, queue);
>> +		/*
>> +		 * Here we only consider PID tracking based on switch events.
>> +		 * For tracking based on CONTEXT packets, the pid is assigned in the function
>> +		 * arm_spe_run_decoder() in order to support timeless decoding.
>> +		 */
>> +		if (!spe->use_ctx_pkt_for_pid)
>> +			arm_spe_set_pid_tid_cpu(spe, queue);
> Yeah, this is right; if without context packet, we need to update thread
> context at this point.
>
> Could you refine the comment like something:
>
> "The switch events has set pid/tid in the machine's context, here we
> update the thread and pid/tid info for spe queue."

Ack.

>
>>  		ret = arm_spe_run_decoder(speq, &ts);
>>  		if (ret < 0) {
>> @@ -740,8 +767,9 @@ static int arm_spe_process_event(struct perf_session *session,
>>  		if (err)
>>  			return err;
>>  
>> -		if (event->header.type == PERF_RECORD_SWITCH_CPU_WIDE ||
>> -		    event->header.type == PERF_RECORD_SWITCH)
>> +		if (!spe->use_ctx_pkt_for_pid &&
>> +		    (event->header.type == PERF_RECORD_SWITCH_CPU_WIDE ||
>> +		     event->header.type == PERF_RECORD_SWITCH))
>>
>>  			err = arm_spe_context_switch(spe, event, sample);
>>  	}
>>  
>> @@ -805,10 +833,16 @@ static int arm_spe_flush(struct perf_session *session __maybe_unused,
>>  		return ret;
>>  
>>  	if (spe->timeless_decoding)
>> -		return arm_spe_process_timeless_queues(spe, -1,
>> +		ret = arm_spe_process_timeless_queues(spe, -1,
>>  				MAX_TIMESTAMP - 1);
>> +	else
>> +		ret = arm_spe_process_queues(spe, MAX_TIMESTAMP);
>>  
>> -	return arm_spe_process_queues(spe, MAX_TIMESTAMP);
>> +	if (!spe->use_ctx_pkt_for_pid)
>> +		ui__warning("Arm SPE CONTEXT packets not found in the traces.\n\n"
>> +			    "Matching of TIDs to SPE events could be inaccurate.\n\n");
> I think we only need to report the warning for no timeless case and
> it's not necessary to change code for timeless decoding, thus the
> change could be:
>
>         ret = arm_spe_process_queues(spe, MAX_TIMESTAMP);
>         if (!spe->use_ctx_pkt_for_pid)
>         ui__warning("Arm SPE CONTEXT packets not found in the traces.\n"
>                     "Matching of TIDs to SPE events could be inaccurate.\n");

Ack (I will also return early if ret != 0).

>
>> +
>> +	return ret;
>>  }
>>  
>>  static void arm_spe_free_queue(void *priv)
>> @@ -1056,6 +1090,22 @@ arm_spe_synth_events(struct arm_spe *spe, struct perf_session *session)
>>  	return 0;
>>  }
>>  
>> +static bool arm_spe_is_ctx_pkt_enabled(struct arm_spe *spe)
>> +{
>> +	struct auxtrace_queues *queues = &spe->queues;
>> +	unsigned int i;
>> +
>> +	for (i = 0; i < queues->nr_queues; i++) {
>> +		struct auxtrace_queue *queue = &spe->queues.queue_array[i];
>> +		struct arm_spe_queue *speq = queue->priv;
>> +
>> +		if (speq)
>> +			return speq->decoder->record.context_id != (u64) -1;
>> +	}
>> +
>> +	return false;
>> +}
>> +
>>  int arm_spe_process_auxtrace_info(union perf_event *event,
>>  				  struct perf_session *session)
>>  {
>> @@ -1131,9 +1181,20 @@ int arm_spe_process_auxtrace_info(union perf_event *event,
>>  	if (err)
>>  		goto err_free_queues;
>>  
>> -	if (spe->queues.populated)
>> +	if (spe->queues.populated) {
>>  		spe->data_queued = true;
>>  
>> +		/*
>> +		 * Ensure the first record of every queue can be read in the function
>> +		 * arm_spe_is_ctx_pkt_enabled()
>> +		 */
>> +		err = arm_spe__update_queues(spe);
>> +		if (err)
>> +			goto err_free_queues;
>> +
>> +		spe->use_ctx_pkt_for_pid = arm_spe_is_ctx_pkt_enabled(spe);
> I don't think this change is needed.
>
> arm_spe__setup_queue() will start to decode and it returns back the
> first record; then function arm_spe_run_decoder() will check
> 'record->context_id', if detect 'context_id' is not '-1' it will set
> 'spe->use_ctx_pkt_for_pid' as true.
>
> So here we don't need to invoke arm_spe__update_queues().  And I have
> some concern that this might introduce other potential issue,
> especially the callback process_auxtrace_info() usually is only used
> for early initializatoin rather than trace data decoding.

Ack, with the above change in arm_spe_run_decoder, this is no longer
needed.

Thanks,
German

>
> Thanks,
> Leo
>
>> +	}
>> +
>>  	return 0;
>>  
>>  err_free_queues:
>> -- 
>> 2.25.1
>>