linux-kernel - Re: [PATCH v6 00/19] Unify vDSOs across more architectures

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Date:   Fri, 14 Jun 2019 14:16:00 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Vincenzo Frascino <vincenzo.frascino@....com>
cc:     Arnd Bergmann <arnd@...db.de>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mips@...r.kernel.org,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Russell King <linux@...linux.org.uk>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Paul Burton <paul.burton@...s.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Mark Salyzyn <salyzyn@...roid.com>,
        Peter Collingbourne <pcc@...gle.com>,
        Shuah Khan <shuah@...nel.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Huw Davies <huw@...eweavers.com>
Subject: Re: [PATCH v6 00/19] Unify vDSOs across more architectures

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.

Thanks,

	tglx