On Tue, Nov 12, 2019 at 2:30 AM Dmitry Safonov <dima@xxxxxxxxxx> wrote: > > From: Andrei Vagin <avagin@xxxxxxxxxx> > > 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@xxxxxxxxxx > > Link: https://criu.org/Time_namespace > Link: https://lists.openvz.org/pipermail/criu/2018-June/041504.html > Signed-off-by: Andrei Vagin <avagin@xxxxxxxxx> > Co-developed-by: Dmitry Safonov <dima@xxxxxxxxxx> > Signed-off-by: Dmitry Safonov <dima@xxxxxxxxxx> > --- > 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?
