linux-kernel - 'branches' perf event mapping differs on ARC and ARM

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Date:   Tue, 27 Nov 2018 14:36:27 +0000
From:   Eugeniy Paltsev <eugeniy.paltsev@...opsys.com>
To:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "jolsa@...hat.com" <jolsa@...hat.com>,
        "acme@...nel.org" <acme@...nel.org>,
        Alexey Brodkin <alexey.brodkin@...opsys.com>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "will.deacon@....com" <will.deacon@....com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        Vineet Gupta <vineet.gupta1@...opsys.com>
Subject: 'branches' perf event mapping differs on ARC and ARM

Hi,

While playing with perf tool on ARMv7 and ARCv2 processors and profiling the
same application I got interesting results. Even if we got pretty
similar total
execution time and instructions number the number of branches on ARC is about
three times more then on ARM.

I dug into architecture
specific perf sources and found that we map different
HW counters into generic 'branches' event on ARC and ARM.
- We use "ijmp" event on ARC which
counts all jump and branch instructions (regardless
of real execution flow - even if no real jump happens)
- We use "pc_write_retired" event on ARM
which counts only taken branches (Instruction
architecturally executed, condition check pass - software change of the PC)

So I was wondering do you know
which approach is correct?



I guess counting all jump and branch instructions is correct because we use
'branches' event value to calculate relative value of 'branch-misses' using

following formula:
----------------------------8----------------------------
branch-misses-ration = 'branch-misses' / 'branches' * 100.0
----------------
------------8----------------------------
And using only taken branches here is incorrect IMHO. So I guess we should
map 'br_immed_retired' instead of
"pc_write_retired" into generic 'branches'
event on ARM.
-- 
 Eugeniy Paltsev