Re: [PATCH] perf top: Use evsel's cpus to replace user_requested_cpus

["Liang, Kan" <kan.liang@linux.intel.com>]

On 2023-12-12 1:49 p.m., Namhyung Kim wrote:
> On Tue, Dec 12, 2023 at 10:31 AM Mark Rutland <mark.rutland@arm.com> wrote:
>>
>> On Tue, Dec 12, 2023 at 10:00:16AM -0800, Ian Rogers wrote:
>>> On Tue, Dec 12, 2023 at 9:23 AM Namhyung Kim <namhyung@kernel.org> wrote:
>>>>
>>>> On Tue, Dec 12, 2023 at 7:56 AM Liang, Kan <kan.liang@linux.intel.com> wrote:
>>>>>
>>>>>
>>>>>
>>>>> On 2023-12-11 4:13 p.m., Arnaldo Carvalho de Melo wrote:
>>>>>> Em Fri, Dec 08, 2023 at 01:08:55PM -0800, kan.liang@linux.intel.com escreveu:
>>>>>>> From: Kan Liang <kan.liang@linux.intel.com>
>>>>>>>
>>>>>>> perf top errors out on a hybrid machine
>>>>>>>  $perf top
>>>>>>>
>>>>>>>  Error:
>>>>>>>  The cycles:P event is not supported.
>>>>>>>
>>>>>>> The user_requested_cpus may contain CPUs that are invalid for a hybrid
>>>>>>> PMU. It causes perf_event_open to fail.
>>>>>>
>>>>>> ?
>>>>>>
>>>>>> All perf top expects is that the "cycles", the most basic one, be
>>>>>> collected, on all CPUs in the system.
>>>>>>
>>>>>
>>>>> Yes, but for hybrid there is no single "cycles" event which can cover
>>>>> all CPUs.
>>>>
>>>> Does that mean the kernel would reject the legacy "cycles" event
>>>> on hybrid CPUs?
>>>
>>> I believe not. When the extended type isn't set on legacy cycles we
>>> often have the CPU and from that can determine the PMU. The issue is
>>> with the -1 any CPU perf_event_open option. As I was told, the PMU the
>>> event is opened on in this case is the first one registered in the
>>> kernel, on Intel hybrid this could be cpu_core or cpu_atom.. but IIRC
>>> it'll probably be cpu_core. On ARM ¯\_(ツ)_/¯.
>>
>> On ARM it'll be essentially the same as on x86: if you open an event with
>> type==PERF_EVENT_TYPE_HARDWARE (without the extended HW type pointing to a
>> specific PMU), and with cpu==-1, it'll go to an arbitrary CPU PMU, whichever
>> happens to be found by perf_init_event() when iterating over the 'pmus' list.
>>
>> If you open an event with type==PERF_EVENT_TYPE_HARDWARE and cpu!=-1, the event
>> will opened on the appropriate CPU PMU, by virtue of being rejected by others
>> when perf_init_event() iterates over the 'pmus' list.
> 
> Ok, that means "cycles" with cpu == -1 would not work well.
>

Unless a PMU is specified.

> I'm curious if it's possible to do some basic work at the event_init()
> like to preserve (common) resource and to do some other work at
> sched to config PMU on the current CPU.  So that users can simply
> use "cycles" or "instructions" for their processes.
> 

The current code treats the hybrid as two standalone PMUs. To preserve
the common resource in the other PMU, I think the only way is to create
an event on the other PMU. It's what perf tool does now. I don't think
we want to move the logic to the kernel.

I think a possible way is to abstract a common PMU (cpu) which only
includes common PMU features. It should be doable, because without the
enabling code of hybrid, the default PMU is the common PMU. But I don't
know how does it coexist with the other hybrid PMUs if we have both
common PMU and hybrid PMUs available? It may just bring more complexity.

Thanks,
Kan