From: Alexandre Belloni <alexandre.belloni@bootlin.com>
To: Lennart Poettering <mzxreary@0pointer.de>
Cc: Arnd Bergmann <arnd@arndb.de>, "Theodore Y. Ts'o" <tytso@mit.edu>,
Linus Torvalds <torvalds@linux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
John Stultz <john.stultz@linaro.org>,
Stephen Boyd <sboyd@kernel.org>,
Florian Weimer <fweimer@redhat.com>,
"H. Peter Anvin" <hpa@zytor.com>,
Palmer Dabbelt <palmer@dabbelt.com>,
Alistair Francis <alistair.francis@wdc.com>,
GNU C Library <libc-alpha@sourceware.org>,
Karel Zak <kzak@redhat.com>,
OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Subject: Re: New kernel interface for sys_tz and timewarp?
Date: Wed, 14 Aug 2019 11:32:08 +0200 [thread overview]
Message-ID: <20190814093208.GG3600@piout.net> (raw)
In-Reply-To: <20190814090936.GB10516@gardel-login>
On 14/08/2019 11:09:36+0200, Lennart Poettering wrote:
> On Mi, 14.08.19 10:31, Arnd Bergmann (arnd@arndb.de) wrote:
>
> > - glibc stops passing the caller timezone argument to the kernel
> > - the distro kernel disables CONFIG_RTC_HCTOSYS,
> > CONFIG_RTC_SYSTOHC and CONFIG_GENERIC_CMOS_UPDATE
>
> What's the benefit of letting userspace do this? It sounds a lot more
> fragile to leave this syncing to userspace if the kernel can do this
> trivially on its own.
>
It does it trivially and badly:
- hctosys will always think the RTC is in UTC so if the RTC is in
local time, you will anyway have up to 12 hours difference until
userspace fixes that.
- the way systohc and hctosys are working will lead up to a 2 second
drift until ntp runs which is an issue for NTP stratum servers. My
tests show that there is a way for userspace to reduce that to tens of
nanoseconds but this means having a one or two seconds delay when setting
and reading the time. I want that to be opt-in.
- the RTC to be used for hctosys and systohc is hardcoded in Kconfig
and distro usually let the default rtc0 but many platforms have a non
functional RTC that ends up being rtc0. I would prefer that to be a
userspace configuration change instead of a kernel configuration change
> IIRC there are uses in kernel that use CLOCK_REALTIME already before
> userspace starts. e.g. iirc networking generally prefers
> CLOCK_REALTIME timestamps over CLOCK_MONOTONIC timestamps
> (i.e. SO_TIMESTAMP and friends are still CLOCK_REALTIME only so far,
> unless I am missing something). If the kernel comes up with a
> CLOCK_REALTIME that starts at 0 this is pretty annoying I
> figure... Hence, so far I suggested to distros to continue turning on
> the options above, and let the kernel do this on its own without
> involving userspace in that.
>
> Lennart
>
> --
> Lennart Poettering, Berlin
--
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com
next prev parent reply other threads:[~2019-08-14 9:32 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-13 9:05 New kernel interface for sys_tz and timewarp? Arnd Bergmann
2019-08-13 17:10 ` John Stultz
2019-08-13 21:45 ` Alexandre Belloni
2019-08-13 17:30 ` Linus Torvalds
2019-08-13 17:49 ` Paul Eggert
2019-08-13 19:31 ` Florian Weimer
2019-08-13 20:04 ` Arnd Bergmann
2019-08-13 21:56 ` Alexandre Belloni
2019-08-14 0:06 ` Theodore Y. Ts'o
2019-08-14 8:31 ` Arnd Bergmann
2019-08-14 9:09 ` Lennart Poettering
2019-08-14 9:32 ` Alexandre Belloni [this message]
2019-08-14 12:15 ` Lennart Poettering
2019-08-19 11:09 ` Karel Zak
2019-08-19 13:43 ` Thomas Gleixner
2019-08-19 13:49 ` Thomas Gleixner
2019-08-20 18:45 ` Alexandre Belloni
2019-08-20 18:47 ` Alexandre Belloni
2019-08-20 18:58 ` Alexandre Belloni
2019-08-27 16:27 ` Arnd Bergmann
2019-08-27 16:31 ` Alexandre Belloni
2019-08-14 16:26 ` David Laight
2019-08-14 16:47 ` hpa
2019-08-15 13:22 ` Arnd Bergmann
2019-08-15 15:05 ` Theodore Y. Ts'o
2019-08-15 15:24 ` hpa
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