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

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

Hi, thanks for looking into it

On 11/11/2021 12:42, Leo Yan wrote:
> On Thu, Nov 11, 2021 at 12:23:08PM +0000, German Gomez wrote:
>> On 11/11/2021 08:30, Leo Yan wrote:
>>> On Wed, Nov 10, 2021 at 11:59:05PM -0800, Namhyung Kim wrote:
> [...]
>
>>>>>>> +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.
> Okay, let me correct myself :)
>
> I checked Intel-pt's implementation, I understand now that we need to 
> distinguish the cases for pid/tid from context switch event and only tid
> from SPE context packet.
>
> Since the context switch event contains the correct pid and tid
> values, we should set both of them, see Intel-PT's implementation [1].
>
> As Namhyung pointed, we need to set pid as '-1' when we only know the
> tid value from SPE context packet; see [2].

I will correct this and resend the patch for SPE.

Thanks,
German

>
> So we should use the same with Intel-pt.
>
> Sorry for I didn't really understand well Namhyung's suggestion and
> thanks you both pointed out the issue.
>
> Leo
>
> P.s. an offline topic is we should send a patch to fix cs-etm issue
> as well [3].
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/perf/util/intel-pt.c#n2920
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/perf/util/intel-pt.c#n2215
> [3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/perf/util/cs-etm.c#n1121