Re: [PATCH V2 00/23] perf intel-pt: Better support for perf record --cpu

[Adrian Hunter <adrian.hunter@intel.com>]

On 8/05/22 18:08, Leo Yan wrote:
> Hi Adrian,
> 
> On Fri, May 06, 2022 at 03:25:38PM +0300, Adrian Hunter wrote:
>> Hi
>>
>> Here are V2 patches to support capturing Intel PT sideband events such as
>> mmap, task, context switch, text poke etc, on every CPU even when tracing
>> selected user_requested_cpus.  That is, when using the perf record -C or
>>  --cpu option.
>>
>> This is needed for:
>> 1. text poke: a text poke on any CPU affects all CPUs
>> 2. tracing user space: a user space process can migrate between CPUs so
>> mmap events that happen on a different CPU can be needed to decode a
>> user_requested_cpus CPU.
>>
>> For example:
>>
>> 	Trace on CPU 1:
>>
>> 	perf record --kcore -C 1 -e intel_pt// &
>>
>> 	Start a task on CPU 0:
>>
>> 	taskset 0x1 testprog &
>>
>> 	Migrate it to CPU 1:
>>
>> 	taskset -p 0x2 <testprog pid>
>>
>> 	Stop tracing:
>>
>> 	kill %1
>>
>> 	Prior to these changes there will be errors decoding testprog
>> 	in userspace because the comm and mmap events for testprog will not
>> 	have been captured.
> 
> Thanks a lot for this patch set, I believe this is a common issue for
> AUX trace (not only for Intel-PT), so I verified this patch set for both
> Arm CoreSight and SPE; unfortunately both cannot see MMAP events for
> migrated task.  I used below commands:
> 
>   # perf record -B -N --no-bpf-event -e cs_etm//u -C 0 -- taskset --cpu-list 1 uname
>   # perf script  --no-itrace --show-mmap-events -C 1 2>/dev/null | grep MMAP | wc -l
>   0
> 
> 
>   # perf record -B -N --no-bpf-event -e arm_spe_0//u -C 0 -- taskset --cpu-list 1 uname
>   # perf script --no-itrace --show-mmap-events -C 1 2>/dev/null | grep MMAP | wc -l
>   0
> 
> I didn't dive into details for this patch set, so I cannot say the
> failure is caused by any issue in this patch set.  But it's definitely
> we need to look into for Arm platforms to root cause what's the reason
> it cannot record MMAP events properly when migrate tasks.  Loop James
> and German for this reason.

You would need the equivalent of patch "perf intel-pt: Track sideband
system-wide when needed" which makes use of new helper
evlist__add_aux_dummy() to set up the dummy event with the option to
make it "system wide".

cs_etm_recording_options() and arm_spe_recording_options() have similar
code.

You will need to decide if it is worth the extra sideband.  I decided
if it became an issue, it could be made optional in the future.