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

Dmitry Vyukov
2020-01-27T14:12:07Z

On Tue, Nov 12, 2019 at 2:30 AM Dmitry Safonov <dima-nzgTgzXrdUbQT0dZR+AlfA@public.gmane.org> wrote:
>
> From: Andrei Vagin <avagin-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>
>
> 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 <avagin-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> Co-developed-by: Dmitry Safonov <dima-nzgTgzXrdUbQT0dZR+AlfA@public.gmane.org>
> Signed-off-by: Dmitry Safonov <dima-nzgTgzXrdUbQT0dZR+AlfA@public.gmane.org>
> ---
> 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

Is it supposed to be kernel/time/namespace.c?