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Subject: Re: [PATCH v6 00/19] Unify vDSOs across more architectures
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-mips@vger.kernel.org,
        "open list:KERNEL SELFTEST FRAMEWORK" 
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        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Russell King <linux@armlinux.org.uk>,
        Ralf Baechle <ralf@linux-mips.org>,
        Paul Burton <paul.burton@mips.com>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Mark Salyzyn <salyzyn@android.com>,
        Peter Collingbourne <pcc@google.com>,
        Shuah Khan <shuah@kernel.org>,
        Dmitry Safonov <0x7f454c46@gmail.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Huw Davies <huw@codeweavers.com>
References: <20190530141531.43462-1-vincenzo.frascino@arm.com>
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 <alpine.DEB.2.21.1906141413070.1722@nanos.tec.linutronix.de>
From:   Vincenzo Frascino <vincenzo.frascino@arm.com>
Message-ID: <6f3bcd07-6eb4-53d6-d209-de42396a4ee2@arm.com>
Date:   Fri, 14 Jun 2019 13:19:58 +0100
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On 6/14/19 1:16 PM, Thomas Gleixner wrote:
> On Tue, 4 Jun 2019, Vincenzo Frascino wrote:
>> On 31/05/2019 09:46, Arnd Bergmann wrote:
>>> One open question I touched in my review is whether we want to
>>> have a vdso version of clock_getres() in all architectures or not.
>>> I'd prefer to leave it out because there is very little advantage to
>>> it over the system call (the results don't change at runtime and
>>> can easily be cached by libc if performance ever matters), and
>>> it takes up a small amount of memory for the implementation.
>>>
>>
>> I thought about it and I ended up with what proposed in this patchset mainly for
>> symmetry across all the architectures since in the end they use the same common
>> code.
>>
>> It seems also that there is some performance impact (i.e.):
>>
>> clock-getres-monotonic:    libc(system call): 296 nsec/call
>> clock-getres-monotonic:    libc(vdso): 5 nsec/call
> 
> clock_getres() is usually not a hot path operation.
> 
>> I agree with you though when you say that caching it in the libc is a
>> possibility to overcome the performance impact.
>>
>>> We shouldn't just need it for consistency because all callers
>>> would require implementing a fallback to the system call
>>> anyway, to deal with old kernels.
> 
> libc has the fallback already. Let's aim for 1:1 replacement of the
> architecture code first and then add the extra bits in separate patches.
>

Ok, thanks Thomas, I will split the patches accordingly.

> Thanks,
> 
> 	tglx
> 

-- 
Regards,
Vincenzo