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

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Date:   Tue, 27 Nov 2018 15:28:25 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Eugeniy Paltsev <eugeniy.paltsev@...opsys.com>,
        "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:     "mark.rutland@....com" <mark.rutland@....com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        Vineet Gupta <vineet.gupta1@...opsys.com>,
        Alexey Brodkin <alexey.brodkin@...opsys.com>,
        "will.deacon@....com" <will.deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "acme@...nel.org" <acme@...nel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        "jolsa@...hat.com" <jolsa@...hat.com>
Subject: Re: 'branches' perf event mapping differs on ARC and ARM

On 27/11/2018 14:36, Eugeniy Paltsev wrote:
> 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.

But register branches (including procedure returns) are also branches, 
so only counting immediate branches would be just as, if not more, 
misleading.

Robin.