linux-kernel - Re: [PATCH v7 00/25] Unify vDSOs across more architectures

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Message-ID: <20190624135055.7020fc2a@donnerap.cambridge.arm.com>
Date:   Mon, 24 Jun 2019 13:50:55 +0100
From:   Andre Przywara <andre.przywara@....com>
To:     Vincenzo Frascino <vincenzo.frascino@....com>
Cc:     <linux-arch@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-mips@...r.kernel.org>,
        <linux-kselftest@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Russell King <linux@...linux.org.uk>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Paul Burton <paul.burton@...s.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        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>,
        Shijith Thotton <sthotton@...vell.com>
Subject: Re: [PATCH v7 00/25] Unify vDSOs across more architectures

On Fri, 21 Jun 2019 10:52:27 +0100
Vincenzo Frascino <vincenzo.frascino@....com> wrote:

Hi,

> vDSO (virtual dynamic shared object) is a mechanism that the Linux
> kernel provides as an alternative to system calls to reduce where
> possible the costs in terms of cycles.
[ ... ]

Some numbers for the ARM(32) part:

I booted my trusted old Calxeda Midway server (Cortex A-15 cores) and ran
the vdsotest benchmark on it. The results are:
(vdso: times, in nsec/call; n/t: "not tested" (=not implemented))
call				5.2-rc3	5.2-rc3-vdso
clock-gettime-monotonic:        147     142
clock-getres-monotonic:         n/t     34
clock-gettime-monotonic-coarse: 90      96
clock-getres-monotonic-coarse:  n/t     36
clock-gettime-monotonic-raw:    431     142
clock-getres-monotonic-raw:     n/t     35
clock-gettime-tai:              598     150
clock-getres-tai:               n/t     34
clock-gettime-boottime:         592     142
clock-getres-boottime:          n/t     34
clock-gettime-realtime:         149     142
clock-getres-realtime:          n/t     34
clock-gettime-realtime-coarse:  86      96
clock-getres-realtime-coarse:   n/t     36
getcpu:                         n/t     n/t
gettimeofday:                   133     110

So there are some minor improvements, two minor regressions, some
significant improvements (factor 3-4), and some dramatic improvements
(where we actually gained VDSO support).
Overall a pretty impressive outcome for an "Odd fixes" architecture,
especially as it should reduce the future maintenance burden.

Cheers,
Andre.