Re: [PATCH] perf/x86/intel/lbr: fix branch type encoding

[Liang, Kan]

On 2022-08-15 3:45 p.m., Stephane Eranian wrote:
> On Sun, Aug 14, 2022 at 12:37 PM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
>>
>>
>>
>> On 2022-08-12 4:16 a.m., Andi Kleen wrote:
>>>
>>>>
>>>> I think the option is to avoid the overhead of disassembling of branch
>>>> instruction. See eb0baf8a0d92 ("perf/core: Define the common branch type
>>>> classification")
>>>> "Since the disassembling of branch instruction needs some overhead,
>>>> a new PERF_SAMPLE_BRANCH_TYPE_SAVE is introduced to indicate if it
>>>> needs to disassemble the branch instruction and record the branch
>>>> type."
>>>
>>>
>>> Thanks for digging it out. So it was only performance.
>>>
>>>>
>>>> I have no idea how big the overhead is. If we can always be benefit from
>>>> the branch type. I guess we can make it default on.
>>>
>>> I thought even arch LBR had one case where it had to disassemble, but
>>> perhaps it's unlikely enough because it's pre filtered. If yes it may be
>>> ok to enable it there unconditionally at the kernel level.
>>>
>>
>> Yes, Arch LBR should have much less overhead than the previous
>> platforms. The most common branches, JCC and near JMP/CALL, are from the
>> HW. Only the other branches, e.g., far call, SYS* etc, which still rely
>> on the SW disassemble. The number of the other branches should not be
>> big. I agree that we should enable the branch type for the Arch LBR
>> unconditionally at the kernel level.
>>
>> Peter? Stephane? What do you think?
>>
>>> Still have to decide if we want older parts to have more overhead by
>>> default. I guess would need some data on that.
>>
> I don't think you want that. It is okay to have it when it is free. Otherwise it
> is best if it remains opt-in.>>
>> The previous LBR already has high overhead. The branch type overhead
>> will make it worse. I think it's better keep it default off. I think we
>> can make it clear in the document that the branch type is only default
>> on for the new platforms with Arch LBR support (12th-Gen+ client or
>> 4th-Gen Xeon+ server).
>>
> I am okay with that.


I see.

I will post a kernel patch to enable the branch type for Arch LBR by
default, and also update the perf tool document as Arnaldo suggested.

Thanks,
Kan