Re: New kernel interface for sys_tz and timewarp?

Alexandre Belloni
2019-08-14T09:32:15Z

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