time_namespaces(7) Miscellaneous Information Manual time_namespaces(7)
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_MONOT-
ONIC_RAW), a nonsettable clock that represents monotonic time since—
as described by POSIX—"some unspecified point in the past".
• CLOCK_BOOTTIME (and likewise CLOCK_BOOTTIME_ALARM), a nonsettable
clock that is identical to CLOCK_MONOTONIC, except that it also in-
cludes 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_gettime(2), clock_nanosleep(2), nanosleep(2),
timer_settime(2), timerfd_settime(2), and /proc/uptime.
Currently, the only way to create a time namespace is by calling un-
share(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 offsets (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 name-
space in which the children of a process will be created. (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-time 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 is a string that identifies the clock whose offsets are be-
ing shown. This field is either monotonic, for CLOCK_MONOTONIC, or
boottime, for 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. The offset-secs value can be negative, subject to restric-
tions noted below; offset-nanosecs is an unsigned value.
In the initial time namespace, the contents of the timens_offsets file
are as follows:
$ cat /proc/self/timens_offsets
monotonic 0 0
boottime 0 0
In a new time namespace that has had no member processes, the clock off-
sets can be modified by writing newline-terminated records of the same
form to the timens_offsets 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 namespace that owns the time name-
space.
Writes to the timens_offsets file can fail with the following errors:
EINVAL An offset-nanosecs value is greater than 999,999,999.
EINVAL A clock-id value is not valid.
EPERM The caller does not have the CAP_SYS_TIME capability.
ERANGE An offset-secs value is out of range. In particular;
• offset-secs can't be set to a value which would make the cur-
rent time on the corresponding clock inside the namespace a
negative value; and
• offset-secs can't be set to a value such that the time on the
corresponding clock inside the namespace would exceed half of
the value of the kernel constant KTIME_SEC_MAX (this limits
the clock value to a maximum of approximately 146 years).
In a new time namespace created by unshare(2), the contents of the
timens_offsets file are inherited from the time namespace of the creat-
ing 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.
For compatibility with the initial implementation, when writing a clock-
id to the /proc/pid/timens_offsets file, the numerical values of the IDs
can be written instead of the symbolic names show above; i.e., 1 instead
of monotonic, and 7 instead of boottime. For readability, the use of
the symbolic names over the numbers is preferred.
The motivation for adding time namespaces was to allow the monotonic and
boot-time clocks to maintain consistent values during container migra-
tion and checkpoint/restore.
EXAMPLES
The following shell session demonstrates the operation of time name-
spaces. We begin by displaying the inode number of the time namespace
of a shell in the initial time namespace:
$ readlink /proc/$$/ns/time
time:[4026531834]
Continuing in the initial time namespace, we display the system uptime
using uptime(1) and use the clock_times example program shown in
clock_getres(2) to display the values of various clocks:
$ uptime --pretty
up 21 hours, 17 minutes
$ ./clock_times
CLOCK_REALTIME : 1585989401.971 (18356 days + 8h 36m 41s)
CLOCK_TAI : 1585989438.972 (18356 days + 8h 37m 18s)
CLOCK_MONOTONIC: 56338.247 (15h 38m 58s)
CLOCK_BOOTTIME : 76633.544 (21h 17m 13s)
We then use unshare(1) to create a time namespace and execute a bash(1)
shell. From the new shell, we use the built-in echo command to write
records to the timens_offsets file adjusting the offset for the
CLOCK_MONOTONIC clock forward 2 days and the offset for the CLOCK_BOOT-
TIME clock forward 7 days:
$ PS1="ns2# " sudo unshare -T -- bash --norc
ns2# echo "monotonic $((2*24*60*60)) 0" > /proc/$$/timens_offsets
ns2# echo "boottime $((7*24*60*60)) 0" > /proc/$$/timens_offsets
Above, we started the bash(1) shell with the --norc option so that no
start-up scripts were executed. This ensures that no child processes
are created from the shell before we have a chance to update the
timens_offsets file.
We then use cat(1) to display the contents of the timens_offsets file.
The execution of cat(1) creates the first process in the new time name-
space, after which further attempts to update the timens_offsets file
produce an error.
ns2# cat /proc/$$/timens_offsets
monotonic 172800 0
boottime 604800 0
ns2# echo "boottime $((9*24*60*60)) 0" > /proc/$$/timens_offsets
bash: echo: write error: Permission denied
Continuing in the new namespace, we execute uptime(1) and the
clock_times example program:
ns2# uptime --pretty
up 1 week, 21 hours, 18 minutes
ns2# ./clock_times
CLOCK_REALTIME : 1585989457.056 (18356 days + 8h 37m 37s)
CLOCK_TAI : 1585989494.057 (18356 days + 8h 38m 14s)
CLOCK_MONOTONIC: 229193.332 (2 days + 15h 39m 53s)
CLOCK_BOOTTIME : 681488.629 (7 days + 21h 18m 8s)
From the above output, we can see that the monotonic and boot-time
clocks have different values in the new time namespace.
Examining the /proc/pid/ns/time and /proc/pid/ns/time_for_children sym-
bolic links, we see that the shell is a member of the initial time name-
space, but its children are created in the new namespace.
ns2# readlink /proc/$$/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), name-
spaces(7), time(7)
Linux man-pages 6.9.1 2024-05-02 time_namespaces(7)
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