linux-kernel - Re: [RFC PATCH] MIPS: Oprofile: Drop support

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Date:   Fri, 4 May 2018 13:59:32 +0100
From:   Matt Redfearn <matt.redfearn@...s.com>
To:     Robert Richter <rric@...nel.org>
CC:     James Hogan <jhogan@...nel.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        <linux-mips@...ux-mips.org>, Huacai Chen <chenhc@...ote.com>,
        <linux-kernel@...r.kernel.org>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        <oprofile-list@...ts.sf.net>
Subject: Re: [RFC PATCH] MIPS: Oprofile: Drop support

Hi Robert,

On 04/05/18 13:27, Robert Richter wrote:
> On 04.05.18 12:03:12, Matt Redfearn wrote:
>>> As said, oprofile version 0.9.x is still available for cpus that do
>>> not support perf. What is the breakage?
>>
>> The breakage I originally set out to fix was the MT support in perf.
>> https://www.linux-mips.org/archives/linux-mips/2018-04/msg00259.html
>>
>> Since the perf code shares so much copied code from oprofile, those same
>> issues exist in oprofile and ought to be addressed. But as newer oprofile
>> userspace does not use the (MIPS) kernel oprofile code, then we could,
>> perhaps, just remove it (as per the RFC). That would break legacy tools
>> (0.9.x) though...
> 
> Those support perf:
> 
>   (CPU_MIPS32 || CPU_MIPS64 || CPU_R10000 || CPU_SB1 || CPU_CAVIUM_OCTEON || CPU_XLP || CPU_LOONGSON3)
> 
> Here is the total list of CPU_*:
> 
>   $ git grep -h config.CPU_ arch/mips/ | sort -u | wc -l
>   79

To be fair, that list for oprofile is not much different:

arch/mips/oprofile/Makefile:

oprofile-$(CONFIG_CPU_MIPS32)		+= op_model_mipsxx.o
oprofile-$(CONFIG_CPU_MIPS64)		+= op_model_mipsxx.o
oprofile-$(CONFIG_CPU_R10000)		+= op_model_mipsxx.o
oprofile-$(CONFIG_CPU_SB1)		+= op_model_mipsxx.o
oprofile-$(CONFIG_CPU_XLR)		+= op_model_mipsxx.o
oprofile-$(CONFIG_CPU_LOONGSON2)	+= op_model_loongson2.o
oprofile-$(CONFIG_CPU_LOONGSON3)	+= op_model_loongson3.o

However, since those are generally CPU families rather than individual 
CPUs, the number of models supported by each framework tells a different 
story:

git grep -h ops.cpu_type arch/mips/oprofile | wc -l
20

git grep -h pmu.name arch/mips/kernel/perf_event* | wc -l
17

The difference is mainly older CPUs - M14Kc, 20K, loongson1, etc. But 
yes you are right dropping it would kill profiling for them - that being 
the case I guess oprofile should remain and instead just remove support 
for the MT capable CPUs (34K, interAptiv) which are all supported by perf.

Thanks,
Matt


> 
> The comparisation might not be accurate, but at least gives a hint
> that there are many cpus not supporting perf. You would drop profiling
> support at al to them.
> 
> If it is too hard to also fix the oprofile code (code duplication
> seems the main issue here), then it would be also ok to blacklist
> newer cpus to enable oprofile kernel code (where it is broken).
> 
> -Robert
>