linux-kernel - Re: New kernel interface for sys

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Message-ID: <20190814090936.GB10516@gardel-login>
Date:   Wed, 14 Aug 2019 11:09:36 +0200
From:   Lennart Poettering <mzxreary@...inter.de>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     "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>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        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 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.

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