linux-kernel - Re: New kernel interface for sys

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Date:   Wed, 14 Aug 2019 11:32:08 +0200
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Lennart Poettering <mzxreary@...inter.de>
Cc:     Arnd Bergmann <arnd@...db.de>, "Theodore Y. Ts'o" <tytso@....edu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        John Stultz <john.stultz@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Florian Weimer <fweimer@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Alistair Francis <alistair.francis@....com>,
        GNU C Library <libc-alpha@...rceware.org>,
        Karel Zak <kzak@...hat.com>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Subject: Re: New kernel interface for sys_tz and timewarp?

On 14/08/2019 11:09:36+0200, Lennart Poettering wrote:
> On Mi, 14.08.19 10:31, Arnd Bergmann (arnd@...db.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