Re: [PATCH V2 3/5] perf mem: Clean up perf_mem_events__name()

Liang, Kan
2023-12-13T16:17:56Z

On 2023-12-13 8:33 a.m., Leo Yan wrote:
>>> I am a bit suspect if we really need the field '.aux_event', the
>>> '.aux_event' field is only used for generating event string.
>> No, it stores the event encoding for the extra event.
>> ARM doesn't need it, so it's 0.
> I searched a bit and confirmed '.aux_event' is only used in
> util/mem-events.c and for 'perf record'.
>
> I failed to connect the code with "it stores the event encoding for the
> extra event". Could you elaborate a bit for this?

The details of the reason of introducing the mem_load aux event can be
found here.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=61b985e3e775a3a75fda04ce7ef1b1aefc4758bc

A mem_load_aux event is a new requirement for SPR. For the other Intel
platforms, a single mem_load event is good enough to collect the data
source information. But for SPR, we have to group both the mem_load
event and the mem_load_aux event when collecting the data source
information. In the group, the mem_load_aux event must be the leader
event. But for the sample read case, only the sampling of the mem_load
make sense. So the is_mem_loads_aux_event() is introduced to switch the
sampling event to the mem_load event. Here is the perf tool patch.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2a57d40832dc8366bc517bcbbfdb1d7fb583735b

The .aux_event is to store the event encoding of the mem_load_aux event.
If it's the leader of a sampling read group, we should use the second
event as a sampling event.

Thanks,
Kan