Discussions around time namespace are there for a long time. The first attempt to implement it was in 2006 by Jeff Dike. From that time, the topic appears on and off in various discussions.
There are two main use cases for time namespaces: 1. change date and time inside a container; 2. adjust clocks for a container restored from a checkpoint. “It seems like this might be one of the last major obstacles keeping migration from being used in production systems, given that not all containers and connections can be migrated as long as a time dependency is capable of messing it up.” (by github.com/dav-ell) The kernel provides access to several clocks: CLOCK_REALTIME, CLOCK_MONOTONIC, CLOCK_BOOTTIME. Last two clocks are monotonous, but the start points for them are not defined and are different for each system. When a container is migrated from one node to another, all clocks have to be restored into consistent states; in other words, they have to continue running from the same points where they have been dumped. The main idea of this patch set is adding per-namespace offsets for system clocks. When a process in a non-root time namespace requests time of a clock, a namespace offset is added to the current value of this clock and the sum is returned. All offsets are placed on a separate page, this allows us to map it as part of VVAR into user processes and use offsets from VDSO calls. Now offsets are implemented for CLOCK_MONOTONIC and CLOCK_BOOTTIME clocks. v3: Major changes: * Simplify two VDSO images by using static_branch() in vclock_gettime() Removes unwanted conflicts with generic VDSO movement patches and simplifies things by dropping too invasive linker magic. As an alternative to static_branch() we tested an attempt to introduce home-made dynamic patching called retcalls: https://github.com/0x7f454c46/linux/commit/4cc0180f6d65 Considering some theoretical problems with toolchains, we decided to go with long well-tested nop-patching in static_branch(). Though, it was needed to provide backend for relative code. * address Thomas' comments. * add sanity checks for offsets: - the current clock time in a namespace has to be in [0, KTIME_MAX / 2). KTIME_MAX is divided by two here to be sure that the KTIME_MAX limit is still unreachable. Link: https://lkml.org/lkml/2018/9/19/950 Link: https://lkml.org/lkml/2019/2/5/867 v2: There are two major changes: * Two versions of the VDSO library to avoid a performance penalty for host tasks outside time namespace (as suggested by Andy and Thomas). As it has been discussed on timens RFC, adding a new conditional branch `if (inside_time_ns)` on VDSO for all processes is undesirable. It will add a penalty for everybody as branch predictor may mispredict the jump. Also there are instruction cache lines wasted on cmp/jmp. Those effects of introducing time namespace are very much unwanted having in mind how much work have been spent on micro-optimisation VDSO code. Addressing those problems, there are two versions of VDSO's .so: for host tasks (without any penalty) and for processes inside of time namespace with clk_to_ns() that subtracts offsets from host's time. * Allow to set clock offsets for a namespace only before any processes appear in it. Now a time namespace looks 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 to create a new time namespaces, set clock offsets and then populate the namespace with processes. Our performance measurements show that the price of VDSO's clock_gettime() in a child time namespace is about 8% with a hot CPU cache and about 90% with a cold CPU cache. There is no performance regression for host processes outside time namespace on those tests. We wrote two small benchmarks. The first one gettime_perf.c calls clock_gettime() in a loop for 3 seconds. It shows us performance with a hot CPU cache (more clock_gettime() cycles - the better): | before | CONFIG_TIME_NS=n | host | inside timens --------|------------|------------------|-------------|------------- cycles | 139887013 | 139453003 | 139899785 | 128792458 diff (%)| 100 | 99.7 | 100 | 92 The second one gettime_perf_cold.c calls rdtsc, clock_gettime(), rdtsc and shows a difference of second and first rdtsc. We call this binary in a loop 1000 times, get 1000 values and calculate MODE for them. It should show us performance with a cold CPU cache (lesser tsc per cycle - the better): | before | CONFIG_TIME_NS=n | host | inside timens --------|------------|------------------|-------------|------------- tsc | 6748 | 6718 | 6862 | 12682 diff (%)| 100 | 99.6 | 101.7 | 188 The numbers gathered on Intel(R) Core(TM) i5-6300U CPU @ 2.40GHz. Cc: Adrian Reber <adr...@lisas.de> Cc: Andrei Vagin <ava...@openvz.org> Cc: Andy Lutomirski <l...@kernel.org> Cc: Arnd Bergmann <a...@arndb.de> Cc: Christian Brauner <christian.brau...@ubuntu.com> Cc: Cyrill Gorcunov <gorcu...@openvz.org> Cc: Dmitry Safonov <0x7f454...@gmail.com> Cc: "Eric W. Biederman" <ebied...@xmission.com> Cc: "H. Peter Anvin" <h...@zytor.com> Cc: Ingo Molnar <mi...@redhat.com> Cc: Jeff Dike <jd...@addtoit.com> Cc: Oleg Nesterov <o...@redhat.com> Cc: Pavel Emelyanov <xe...@virtuozzo.com> Cc: Shuah Khan <sh...@kernel.org> Cc: Thomas Gleixner <t...@linutronix.de> Cc: Vincenzo Frascino <vincenzo.frasc...@arm.com> Cc: contain...@lists.linux-foundation.org Cc: c...@openvz.org Cc: linux-...@vger.kernel.org Cc: x...@kernel.org v2: https://lore.kernel.org/lkml/20190206001107.16488-1-d...@arista.com/ RFC: https://lkml.kernel.org/r/20180919205037.9574-1-d...@arista.com/ Andrei Vagin (16): ns: Introduce Time Namespace timens: Add timens_offsets timens: Introduce CLOCK_MONOTONIC offsets timens: Introduce CLOCK_BOOTTIME offset timerfd/timens: Take into account ns clock offsets posix-timers/timens: Take into account clock offsets timens/kernel: Take into account timens clock offsets in clock_nanosleep x86/vdso: Add offsets page in vvar vdso: introduce timens_static_branch timens/fs/proc: Introduce /proc/pid/timens_offsets selftest/timens: Add a test for timerfd selftest/timens: Add a test for clock_nanosleep() selftest/timens: Add timer offsets test x86/vdso: Align VDSO functions by CPU L1 cache line selftests: Add a simple perf test for clock_gettime() selftest/timens: Check that a right vdso is mapped after fork and exec Dmitry Safonov (11): timens: Shift /proc/uptime x86/vdso2c: Correct err messages on file opening x86/vdso2c: Convert iterator to unsigned x86/vdso/Makefile: Add vobjs32 x86/vdso: Restrict splitting VVAR VMA x86/vdso: Rename vdso_image {.data=>.text} x86/vdso: Allocate timens vdso x86/vdso: Switch image on setns()/unshare()/clone() timens: Add align for timens_offsets selftest/timens: Add Time Namespace test for supported clocks selftest/timens: Add procfs selftest MAINTAINERS | 3 + arch/Kconfig | 5 + arch/x86/Kconfig | 1 + arch/x86/entry/vdso/Makefile | 16 +- arch/x86/entry/vdso/vclock_gettime.c | 48 +++ arch/x86/entry/vdso/vdso-layout.lds.S | 10 +- arch/x86/entry/vdso/vdso2c.c | 7 +- arch/x86/entry/vdso/vdso2c.h | 24 +- arch/x86/entry/vdso/vma.c | 113 ++++++- arch/x86/include/asm/jump_label.h | 14 + arch/x86/include/asm/vdso.h | 14 +- fs/proc/base.c | 101 ++++++ fs/proc/namespaces.c | 4 + fs/proc/uptime.c | 3 + fs/timerfd.c | 8 +- include/linux/hrtimer.h | 1 + include/linux/jump_label.h | 5 + include/linux/nsproxy.h | 2 + include/linux/proc_ns.h | 2 + include/linux/time_namespace.h | 137 ++++++++ include/linux/timens_offsets.h | 18 + include/linux/user_namespace.h | 1 + include/uapi/linux/sched.h | 1 + include/uapi/linux/time.h | 2 + init/Kconfig | 8 + kernel/Makefile | 1 + kernel/fork.c | 3 +- kernel/nsproxy.c | 41 ++- kernel/time/alarmtimer.c | 13 +- kernel/time/hrtimer.c | 3 + kernel/time/posix-stubs.c | 11 +- kernel/time/posix-timers.c | 27 +- kernel/time_namespace.c | 318 ++++++++++++++++++ tools/testing/selftests/Makefile | 1 + tools/testing/selftests/timens/.gitignore | 8 + tools/testing/selftests/timens/Makefile | 12 + .../selftests/timens/clock_nanosleep.c | 100 ++++++ tools/testing/selftests/timens/config | 1 + tools/testing/selftests/timens/exec.c | 91 +++++ tools/testing/selftests/timens/gettime_perf.c | 74 ++++ .../selftests/timens/gettime_perf_cold.c | 63 ++++ tools/testing/selftests/timens/log.h | 26 ++ tools/testing/selftests/timens/procfs.c | 142 ++++++++ tools/testing/selftests/timens/timens.c | 188 +++++++++++ tools/testing/selftests/timens/timens.h | 63 ++++ tools/testing/selftests/timens/timer.c | 116 +++++++ tools/testing/selftests/timens/timerfd.c | 127 +++++++ 47 files changed, 1928 insertions(+), 49 deletions(-) create mode 100644 include/linux/time_namespace.h create mode 100644 include/linux/timens_offsets.h create mode 100644 kernel/time_namespace.c create mode 100644 tools/testing/selftests/timens/.gitignore create mode 100644 tools/testing/selftests/timens/Makefile create mode 100644 tools/testing/selftests/timens/clock_nanosleep.c create mode 100644 tools/testing/selftests/timens/config create mode 100644 tools/testing/selftests/timens/exec.c create mode 100644 tools/testing/selftests/timens/gettime_perf.c create mode 100644 tools/testing/selftests/timens/gettime_perf_cold.c create mode 100644 tools/testing/selftests/timens/log.h create mode 100644 tools/testing/selftests/timens/procfs.c create mode 100644 tools/testing/selftests/timens/timens.c create mode 100644 tools/testing/selftests/timens/timens.h create mode 100644 tools/testing/selftests/timens/timer.c create mode 100644 tools/testing/selftests/timens/timerfd.c -- 2.21.0