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

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Date:   Fri, 12 Aug 2022 10:16:50 +0200
From:   Andi Kleen <ak@...ux.intel.com>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>,
        Stephane Eranian <eranian@...gle.com>
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        kan.liang@...el.com, acme@...hat.com, namhyung@...nel.org,
        irogers@...gle.com
Subject: Re: [PATCH] perf/x86/intel/lbr: fix branch type encoding


>
> 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.

Still have to decide if we want older parts to have more overhead by 
default. I guess would need some data on that.


-Andi