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

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Message-ID: <20190620172746.7f92cb14@donnerap.cambridge.arm.com>
Date:   Thu, 20 Jun 2019 17:27:46 +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, Shuah Khan <shuah@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Huw Davies <huw@...eweavers.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Will Deacon <will.deacon@....com>,
        Russell King <linux@...linux.org.uk>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Mark Salyzyn <salyzyn@...roid.com>,
        Paul Burton <paul.burton@...s.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Collingbourne <pcc@...gle.com>
Subject: Re: [PATCH v6 00/19] Unify vDSOs across more architectures

On Thu, 30 May 2019 15:15:12 +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.
> [ ... ]
> The porting has been benchmarked and the performance results are
> provided as part of this cover letter.

I can't reveal the absolute numbers here, but vdsotest-bench gives me
quite some performance gain on my board here ("time needed on v6" divided
by "time needed on 5.2-rc1", so smaller percentages are better):
clock-gettime-monotonic:	23 %
clock-gettime-monotonic-raw:	30 %
clock-gettime-tai:		 5 %
clock-getres-tai:		 5 %
clock-gettime-boottime:		 5 %
clock-getres-boottime:		 5 %
clock-gettime-realtime:		25 %
gettimeofday:			26 % 
The other numbers stayed the same or differed by just 1 ns, which seems to
be within the margin of error, as repeated runs on the same kernel suggest.
The 5% numbers are of course those were we went from a syscall-only to the
newly added arm64 VDSO implementation, but even the other calls improved
by a factor of 3 or more.

Sounds like a strong indicator that this is a good thing to have.

Not sure if "running some benchmark a couple of times on a single machine"
qualifies for this, but I guess it means:

Tested-by: Andre Przywara <andre.przywara@....com>

Cheers,
Andre.