Messages in this thread | | | Date | Mon, 15 Aug 2022 16:39:13 -0400 | Subject | Re: [PATCH] perf/x86/intel/lbr: fix branch type encoding | From | "Liang, Kan" <> |
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On 2022-08-15 3:45 p.m., Stephane Eranian wrote: > On Sun, Aug 14, 2022 at 12:37 PM Liang, Kan <kan.liang@linux.intel.com> 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
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