linux-kernel - Re: RFC: time_namespaces(7) manual page

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Date:   Mon, 6 Apr 2020 20:23:18 -0700
From:   Andrei Vagin <avagin@...il.com>
To:     "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:     Andrei Vagin <avagin@...nvz.org>, Dmitry Safonov <dima@...sta.com>,
        linux-man <linux-man@...r.kernel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Linux API <linux-api@...r.kernel.org>,
        Containers <containers@...ts.linux-foundation.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        lkml <linux-kernel@...r.kernel.org>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Adrian Reber <adrian@...as.de>
Subject: Re: 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