RFC: time_namespaces(7) manual page
From: Michael Kerrisk (man-pages)
Date: Sat Apr 04 2020 - 07:18:09 EST
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 monoâ
tonic time sinceâas described by POSIXâ"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.
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 offâ
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.
/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_BOOTâ
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:
$ cat /proc/self/timens_offsets
1 0 0
7 0 0
In a new time namespace that has had no member processes, the
clock offsets can be modified by writing newline-terminated
records of the same form to the file. The file can be written to
multiple times, but after the first process has been created in or
has entered the namespace, write(2)s on this file fail with the
error EACCES. In order to write to the timens_offsets file, a
process must have the CAP_SYS_TIME capability in the user namesâ
pace that owns the time namespace.
In a new time namespace created by unshare(2), the contents of the
timens_offsets file are inherited from the time namespace of the
creating process.
NOTES
Use of time namespaces requires a kernel that is configured with
the CONFIG_TIME_NS option.
Note that time namespaces do not virtualize the CLOCK_REALTIME
clock. Virtualization of this clock was avoided for reasons of
complexity and overhead within the kernel.
The motivation for adding time namespaces was to allow the monoâ
tonic and boot-time clocks to maintain consistent values during
container migration and checkpoint/restore.
EXAMPLE
The following shell session demonstrates the operation of time
namespaces. We begin by displaying the inode number of the time
namespace of a shell in the initial time namespace:
$ readlink /proc/$$/ns/time
time:[4026531834]
Continuing in the initial time namespace, we display the system
uptime using uptime(1) and use the clock_times example program
shown in clock_getres(2) to display the values of various clocks:
$ uptime --pretty
up 21 hours, 17 minutes
$ ./clock_times
CLOCK_REALTIME : 1585989401.971 (18356 days + 8h 36m 41s)
CLOCK_TAI : 1585989438.972 (18356 days + 8h 37m 18s)
CLOCK_MONOTONIC: 56338.247 (15h 38m 58s)
CLOCK_BOOTTIME : 76633.544 (21h 17m 13s)
We then use unshare(1) to create a time namespace and execute a
bash(1) shell. From the new shell, we use the built-in echo comâ
mand to write records to the timens_offsets file adjusting the
offset for the CLOCK_MONOTONIC clock forward 2 days and the offset
for the CLOCK_BOOTTIME clock forward 7 days:
$ PS1="ns2# " sudo unshare -T -- bash --norc
ns2# echo "1 $((2*24*60*60)) 0" > /proc/$$/timens_offsets
ns2# echo "7 $((7*24*60*60)) 0" > /proc/$$/timens_offsets
Above, we started the bash(1) shell with the --norc options so
that no start-up scripts were executed. This ensures that no
child processes are created from the shell before we have a chance
to update the timens_offsets file.
We then use cat(1) to display the contents of the timens_offsets
file. The execution of cat(1) creates the first process in the
new time namespace, after which further attempts to update the
timens_offsets file produce an error.
ns2# cat /proc/$$/timens_offsets
1 172800 0
7 604800 0
ns2# echo "7 $((9*24*60*60)) 0" > /proc/$$/timens_offsets
bash: echo: write error: Permission denied
Continuing in the new namespace, we execute uptime(1) and the
clock_times example program:
ns2# uptime --pretty
up 1 week, 21 hours, 18 minutes
ns2# ./clock_times
CLOCK_REALTIME : 1585989457.056 (18356 days + 8h 37m 37s)
CLOCK_TAI : 1585989494.057 (18356 days + 8h 38m 14s)
CLOCK_MONOTONIC: 229193.332 (2 days + 15h 39m 53s)
CLOCK_BOOTTIME : 681488.629 (7 days + 21h 18m 8s)
From the above output, we can see that the monotonic and boot-time
clocks have new values in the new time namespace.
Examining the /proc/[pid]/ns/time and
/proc/[pid]/ns/time_for_children symbolic links, we see that the
shell is a member of the initial time namespace, but its children
are created in a different namespace.
ns2# readlink /proc/$$/ns/time
time:[4026531834]
ns2# readlink /proc/$$/ns/time_for_children
time:[4026532900]
ns2# readlink /proc/self/ns/time # Creates a child process
time:[4026532900]
Returning to the shell in the initial time namespace, we see that
the monotonic and boot-time clocks are unaffected by the
timens_offsets changes that were made in the other time namespace:
$ uptime --pretty
up 21 hours, 19 minutes
$ ./clock_times
CLOCK_REALTIME : 1585989401.971 (18356 days + 8h 38m 51s)
CLOCK_TAI : 1585989438.972 (18356 days + 8h 39m 28s)
CLOCK_MONOTONIC: 56338.247 (15h 41m 8s)
CLOCK_BOOTTIME : 76633.544 (21h 19m 23s)
SEE ALSO
nsenter(1), unshare(1), clock_settime(2). setns(2), unshare(2),
namespaces(7), time(7)
Linux 2020-04-01 TIME_NAMESPACES(7)
========== Page source below ==========
.\" Copyright (c) 2020 by Michael Kerrisk <mtk.manpages@xxxxxxxxx>
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.TH TIME_NAMESPACES 7 2020-04-01 "Linux" "Linux Programmer's Manual"
.SH NAME
time_namespaces \- overview of Linux time namespaces
.SH DESCRIPTION
Time namespaces virtualize the values of two system clocks:
.IP \(bu 2
.BR CLOCK_MONOTONIC
(and likewise
.BR CLOCK_MONOTONIC_COARSE
and
.BR CLOCK_MONOTONIC_RAW ),
a nonsettable clock that represents monotonic time since\(emas
described by POSIX\(em"some unspecified point in the past".
.IP \(bu
.BR CLOCK_BOOTTIME
(and likewise
.BR CLOCK_BOOTTIME_ALARM ),
a clock that is identical to
.BR CLOCK_MONOTONIC ,
except that it also includes any time that the system is suspended.
.PP
Thus, the processes in a time namespace share per-namespace values
for these clocks.
This affects various APIs that measure against these clocks, including:
.BR clock_nanosleep (2),
.BR nanosleep (2),
.BR clock_gettime (2),
and
.IR /proc/uptime .
.PP
Currently, the only way to create a time namespace is by calling
.BR unshare (2)
with the
.BR CLONE_NEWTIME
flag.
This call creates a new time namespace but does
.I 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 offsets (see below) for the new namespace
to be set before the first process is placed in the namespace.
The
.IR /proc/[pid]/ns/time_for_children
symbolic link shows the time namespace in which
the children of a process will be created.
.\"
.SS /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
.IR /proc/PID/timens_offsets .
Within this file,
the offsets are expressed as lines consisting of
three space-delimited fields:
.PP
.in +4n
.EX
<clock-id> <offset-secs> <offset-nanosecs>
.EE
.in
.PP
The
.I clock-id
identifies the clock whose offsets are being shown.
This field is either 1, for
.BR CLOCK_MONOTONIC ,
or 7, for
.BR CLOCK_BOOTTIME .
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:
.PP
.in +4n
.EX
$ \fBcat /proc/self/timens_offsets\fP
1 0 0
7 0 0
.EE
.in
.PP
In a new time namespace that has had no member processes,
the clock offsets can be modified by writing newline-terminated
records of the same form to the file.
The file can be written to multiple times,
but after the first process has been created in or has entered the namespace,
.BR write (2)s
on this file fail with the error
.BR EACCES .
In order to write to the
.IR timens_offsets
file, a process must have the
.BR CAP_SYS_TIME
capability in the user namespace that owns the time namespace.
.PP
In a new time namespace created by
.BR unshare (2),
the contents of the
.I timens_offsets
file are inherited from the time namespace of the creating process.
.SH NOTES
.PP
Use of time namespaces requires a kernel that is configured with the
.B CONFIG_TIME_NS
option.
.PP
Note that time namespaces do not virtualize the
.BR CLOCK_REALTIME
clock.
Virtualization of this clock was avoided for reasons of complexity
and overhead within the kernel.
.PP
The motivation for adding time namespaces was to allow
the monotonic and boot-time clocks to maintain consistent values
during container migration and checkpoint/restore.
.SH EXAMPLE
.PP
The following shell session demonstrates the operation of time namespaces.
We begin by displaying the inode number of the time namespace
of a shell in the initial time namespace:
.PP
.in +4n
.EX
$ \fBreadlink /proc/$$/ns/time\fP
time:[4026531834]
.EE
.in
.PP
Continuing in the initial time namespace, we display the system uptime using
.BR uptime (1)
and use the
.I clock_times
example program shown in
.BR clock_getres (2)
to display the values of various clocks:
.PP
.in +4n
.EX
$ \fBuptime \-\-pretty\fP
up 21 hours, 17 minutes
$ \fB./clock_times\fP
CLOCK_REALTIME : 1585989401.971 (18356 days + 8h 36m 41s)
CLOCK_TAI : 1585989438.972 (18356 days + 8h 37m 18s)
CLOCK_MONOTONIC: 56338.247 (15h 38m 58s)
CLOCK_BOOTTIME : 76633.544 (21h 17m 13s)
.EE
.in
.PP
We then use
.BR unshare (1)
to create a time namespace and execute a
.BR bash (1)
shell.
>From the new shell, we use the built-in
.B echo
command to write records to the
.I timens_offsets
file adjusting the offset for the
.B CLOCK_MONOTONIC
clock forward 2 days
and the offset for the
.B CLOCK_BOOTTIME
clock forward 7 days:
.PP
.in +4n
.EX
$ \fBPS1="ns2# " sudo unshare \-T \-\- bash \-\-norc\fP
ns2# \fBecho "1 $((2*24*60*60)) 0" > /proc/$$/timens_offsets\fP
ns2# \fBecho "7 $((7*24*60*60)) 0" > /proc/$$/timens_offsets\fP
.EE
.in
.PP
Above, we started the
.BR bash (1)
shell with the
.B \-\-norc
options so that no start-up scripts were executed.
This ensures that no child processes are created from the
shell before we have a chance to update the
.I timens_offsets
file.
.PP
We then use
.BR cat (1)
to display the contents of the
.I timens_offsets
file.
The execution of
.BR cat (1)
creates the first process in the new time namespace,
after which further attempts to update the
.I timens_offsets
file produce an error.
.PP
.in +4n
.EX
ns2# \fBcat /proc/$$/timens_offsets\fP
1 172800 0
7 604800 0
ns2# \fBecho "7 $((9*24*60*60)) 0" > /proc/$$/timens_offsets\fP
bash: echo: write error: Permission denied
.EE
.in
.PP
Continuing in the new namespace, we execute
.BR uptime (1)
and the
.I clock_times
example program:
.PP
.in +4n
.EX
ns2# \fBuptime \-\-pretty\fP
up 1 week, 21 hours, 18 minutes
ns2# \fB./clock_times\fP
CLOCK_REALTIME : 1585989457.056 (18356 days + 8h 37m 37s)
CLOCK_TAI : 1585989494.057 (18356 days + 8h 38m 14s)
CLOCK_MONOTONIC: 229193.332 (2 days + 15h 39m 53s)
CLOCK_BOOTTIME : 681488.629 (7 days + 21h 18m 8s)
.EE
.in
.PP
>From the above output, we can see that the monotonic
and boot-time clocks have new values in the new time namespace.
.PP
Examining the
.I /proc/[pid]/ns/time
and
.I /proc/[pid]/ns/time_for_children
symbolic links, we see that the shell is a member of the initial time
namespace, but its children are created in a different namespace.
.PP
.in +4n
.EX
ns2# \fBreadlink /proc/$$/ns/time\fP
time:[4026531834]
ns2# \fBreadlink /proc/$$/ns/time_for_children\fP
time:[4026532900]
ns2# \fBreadlink /proc/self/ns/time\fP # Creates a child process
time:[4026532900]
.EE
.in
.PP
Returning to the shell in the initial time namespace,
we see that the monotonic and boot-time clocks
are unaffected by the
.I timens_offsets
changes that were made in the other time namespace:
.PP
.in +4n
.EX
$ \fBuptime \-\-pretty\fP
up 21 hours, 19 minutes
$ \fB./clock_times\fP
CLOCK_REALTIME : 1585989401.971 (18356 days + 8h 38m 51s)
CLOCK_TAI : 1585989438.972 (18356 days + 8h 39m 28s)
CLOCK_MONOTONIC: 56338.247 (15h 41m 8s)
CLOCK_BOOTTIME : 76633.544 (21h 19m 23s)
.EE
.in
.SH SEE ALSO
.BR nsenter (1),
.BR unshare (1),
.BR clock_settime (2).
.\" clone3() support for time namespaces is a work in progress
.\" .BR clone3 (2),
.BR setns (2),
.BR unshare (2),
.BR namespaces (7),
.BR time (7)
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/