Re: [PATCHv8 03/34] ns: Introduce Time Namespace

From: Andrei Vagin <redacted>

Time Namespace isolates clock values.

The kernel provides access to several clocks CLOCK_REALTIME,
CLOCK_MONOTONIC, CLOCK_BOOTTIME, etc.

CLOCK_REALTIME
      System-wide clock that measures real (i.e., wall-clock) time.

CLOCK_MONOTONIC
      Clock that cannot be set and represents monotonic time since
      some unspecified starting point.

CLOCK_BOOTTIME
      Identical to CLOCK_MONOTONIC, except it also includes any time
      that the system is suspended.

For many users, the time namespace means the ability to changes date and
time in a container (CLOCK_REALTIME).

But in a context of the checkpoint/restore functionality, monotonic and
bootime clocks become interesting. Both clocks are monotonic with
unspecified staring points. These clocks are widely used to measure time
slices and set timers. After restoring or migrating processes, we have to
guarantee that they never go backward. In an ideal case, the behavior of
these clocks should be the same as for a case when a whole system is
suspended. All this means that we need to be able to set CLOCK_MONOTONIC
and CLOCK_BOOTTIME clocks, what can be done by adding per-namespace
offsets for clocks.

A time namespace is similar to a pid namespace in a way how it is
created: unshare(CLONE_NEWTIME) system call creates a new time namespace,
but doesn't set it to the current process. Then all children of
the process will be born in the new time namespace, or a process can
use the setns() system call to join a namespace.

This scheme allows setting clock offsets for a namespace, before any
processes appear in it.

All available clone flags have been used, so CLONE_NEWTIME uses the
highest bit of CSIGNAL. It means that we can use it with the unshare()
system call only. Rith now, this works for us, because time namespace
offsets can be set only when a new time namespace is not populated. In a
future, we will have the clone3() system call [1] which will allow to use
the CSIGNAL mask for clone flags.

[1]: httmps://lkml.kernel.org/r/20190604160944.4058-1-christian-STijNZzMWpgWenYVfaLwtA@public.gmane.org

Link: https://criu.org/Time_namespace
Link: https://lists.openvz.org/pipermail/criu/2018-June/041504.html
Signed-off-by: Andrei Vagin <redacted>
Co-developed-by: Dmitry Safonov <redacted>
Signed-off-by: Dmitry Safonov <redacted>
---
 MAINTAINERS                    |   2 +
 fs/proc/namespaces.c           |   4 +
 include/linux/nsproxy.h        |   2 +
 include/linux/proc_ns.h        |   3 +
 include/linux/time_namespace.h |  66 ++++++++++
 include/linux/user_namespace.h |   1 +
 include/uapi/linux/sched.h     |   6 +
 init/Kconfig                   |   7 ++
 kernel/fork.c                  |  16 ++-
 kernel/nsproxy.c               |  41 +++++--
 kernel/time/Makefile           |   1 +
 kernel/time/namespace.c        | 217 +++++++++++++++++++++++++++++++++
 12 files changed, 356 insertions(+), 10 deletions(-)
 create mode 100644 include/linux/time_namespace.h
 create mode 100644 kernel/time/namespace.c

diff --git a/MAINTAINERS b/MAINTAINERS
index 3f7f8cdbc471..037abc28c414 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS

@@ -13172,6 +13172,8 @@ T:      git git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git timers/core
 S:     Maintained
 F:     fs/timerfd.c
 F:     include/linux/timer*
+F:     include/linux/time_namespace.h
+F:     kernel/time_namespace.c