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SubjectRe: RFC: time_namespaces(7) manual page
Hi Michael,

The man page looks good to me. A few comments are inline.

On Sat, Apr 04, 2020 at 01:08:50PM +0200, Michael Kerrisk (man-pages) wrote:
> Hello Dmitry, Andrei, et al.
>
> I have written a manual page to document time namespaces.
> Could you please take a look and let me know of any
> corrections, improvements, etc.
>
> The rendered page is shown below. Th epage source is at the foot of
> this mail.
>
> Thanks,
>
> Michael
>
>
> NAME
> time_namespaces - overview of Linux time namespaces
>
> DESCRIPTION
> Time namespaces virtualize the values of two system clocks:
>
> · CLOCK_MONOTONIC (and likewise CLOCK_MONOTONIC_COARSE and
> CLOCK_MONOTONIC_RAW), a nonsettable clock that represents mono‐
> tonic time since—as described by POSIX—"some unspecified
> point in the past".
>
> · CLOCK_BOOTTIME (and likewise CLOCK_BOOTTIME_ALARM), a clock that
> is identical to CLOCK_MONOTONIC, except that it also includes
> any time that the system is suspended.
>
> Thus, the processes in a time namespace share per-namespace values
> for these clocks. This affects various APIs that measure against
> these clocks, including: clock_nanosleep(2), nanosleep(2),
> clock_gettime(2), and /proc/uptime.

timer_settime, timerfd_settime

>
> Currently, the only way to create a time namespace is by calling
> unshare(2) with the CLONE_NEWTIME flag. This call creates a new
> time namespace but does not place the calling process in the new
> namespace. Instead, the calling process's subsequently created
> children are placed in the new namespace. This allows clock off‐
> sets (see below) for the new namespace to be set before the first
> process is placed in the namespace. The
> /proc/[pid]/ns/time_for_children symbolic link shows the time
> namespace in which the children of a process will be created.

We can mention that the current process can enter the namespace if it
call setns on /proc/self/ns/time_for_children.

>
> /proc/PID/timens_offsets
> Associated with each time namespace are offsets, expressed with
> respect to the initial time namespace, that define the values of
> the monotonic and boot clocks in that namespace. These offsets
> are exposed via the file /proc/PID/timens_offsets. Within this
> file, the offsets are expressed as lines consisting of three
> space-delimited fields:
>
> <clock-id> <offset-secs> <offset-nanosecs>
>
> The clock-id identifies the clock whose offsets are being shown.
> This field is either 1, for CLOCK_MONOTONIC, or 7, for CLOCK_BOOT‐
> TIME. The remaining fields express the offset (seconds plus
> nanoseconds) for the clock in this time namespace. These offsets
> are expressed relative to the clock values in the initial time
> namespace. In the initial time namespace, the contents of this
> file are as follows:

I think we can mention that offset-secs can be negative, but
offset-nanosleep has to be 0 or positive.

Thanks,
Andrei

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