RFC: time_namespaces(7) manual page

[Michael Kerrisk (man-pages)]

Hello Andrei,

On 4/7/20 5:23 AM, Andrei Vagin wrote:
> Hi Michael,
> 
> The man page looks good to me. A few comments are inline.

Thanks for looking the page over!

> 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

Added.

>>        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.

Yes, thanks. I overlooked that, and I was puzzled about how setns() could
be useful before the first process was created in a time NS.

I added:

    (A process can use a file descriptor opened on this symbolic
    link in a call to setns(2) in order to move into the namespace.)

>>    /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. See my upcoming reply to Thomas.

Cheers,

Michael



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/