Re: [PATCH v2 4/4] perf arm-spe: Support hardware-based PID tracing

[German Gomez <german.gomez@arm.com>]

Hi Leo, Namhyung,

On 11/11/2021 08:30, Leo Yan wrote:
> Hi Namhyung,
>
> On Wed, Nov 10, 2021 at 11:59:05PM -0800, Namhyung Kim wrote:
>
> [...]
>
>>>>> +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);
>>>> I think we should pass -1 as pid as we don't know the real pid.
>>> AFAICT, I observe one case for machine__set_current_tid() returning error
>>> is 'speq->cpu' is -1 (this is the case for per-thread tracing).  In
>>> this case, if pass '-1' for pid/tid, it still will return failure.
>>>
>>> So here should return the error as it is.  Am I missing anything?
>> I'm not saying about the error.  It's about thread status.
>> In the machine__set_current_tid(), it calls
>> machine__findnew_thread() with given pid and tid.
>>
>> I suspect it can set pid to a wrong value if the thread has
>> no pid value at the moment.
> Here we should avoid to write pid '-1' with
> machine__set_current_tid().

If the kernel is writing the tids to the contextidr, isn't it wrong to
assume tid == pid when decoding the context packets here? I haven't
observed any impact in the built-in commands though, so there must be
something I'm not seeing.

Thanks,
German
>
> The function arm_spe_set_tid() is invoked when SPE trace data contains
> context packet and it passes pid coming from the context packet.  On
> the other hand, when SPE trace data doesn't contain context packet, we
> relies on context switch event to set pid value.  So if we pass pid
> '-1' in arm_spe_set_tid(), it will overwrite the pid value which has
> been set by context switch event.
>
> Simply say, if SPE trace data contains context packet with valid pid,
> perf invokes arm_spe_set_tid() to set the pid value.  Otherwise, it
> should skip this operation and roll back to use the pid value from
> the context switch event.
>
> Thanks,
> Leo