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Currently, MAX_SKEW_USEC is set to 100 microseconds, which has worked reasonably well. However, NTP is willing to tolerate 500 microseconds of skew per second, and a clocksource that is good enough for NTP should be good enough for the clocksource watchdog. The watchdog's skew is controlled by MAX_SKEW_USEC and the CLOCKSOURCE_WATCHDOG_MAX_SKEW_US Kconfig option. However, these values are doubled before being associated with a clocksource's ->uncertainty_margin, and the ->uncertainty_margin values of the pair of clocksource's being compared are summed before checking against the skew. Therefore, set both MAX_SKEW_USEC and the default for the CLOCKSOURCE_WATCHDOG_MAX_SKEW_US Kconfig option to 125 microseconds of skew per second, resulting in 500 microseconds of skew per second in the clocksource watchdog's skew comparison. Suggested-by Rik van Riel <riel@surriel.com> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
6.1 KiB
SPDX-License-Identifier: GPL-2.0-only
Timer subsystem related configuration options
Options selectable by arch Kconfig
Watchdog function for clocksources to detect instabilities
config CLOCKSOURCE_WATCHDOG bool
Architecture has extra clocksource data
config ARCH_CLOCKSOURCE_DATA bool
Architecture has extra clocksource init called from registration
config ARCH_CLOCKSOURCE_INIT bool
Clocksources require validation of the clocksource against the last
cycle update - x86/TSC misfeature
config CLOCKSOURCE_VALIDATE_LAST_CYCLE bool
Timekeeping vsyscall support
config GENERIC_TIME_VSYSCALL bool
The generic clock events infrastructure
config GENERIC_CLOCKEVENTS def_bool !LEGACY_TIMER_TICK
Architecture can handle broadcast in a driver-agnostic way
config ARCH_HAS_TICK_BROADCAST bool
Clockevents broadcasting infrastructure
config GENERIC_CLOCKEVENTS_BROADCAST bool depends on GENERIC_CLOCKEVENTS
Automatically adjust the min. reprogramming time for
clock event device
config GENERIC_CLOCKEVENTS_MIN_ADJUST bool
Generic update of CMOS clock
config GENERIC_CMOS_UPDATE bool
Select to handle posix CPU timers from task_work
and not from the timer interrupt context
config HAVE_POSIX_CPU_TIMERS_TASK_WORK bool
config POSIX_CPU_TIMERS_TASK_WORK bool default y if POSIX_TIMERS && HAVE_POSIX_CPU_TIMERS_TASK_WORK
config LEGACY_TIMER_TICK bool help The legacy timer tick helper is used by platforms that lack support for the generic clockevent framework. New platforms should use generic clockevents instead.
config TIME_KUNIT_TEST tristate "KUnit test for kernel/time functions" if !KUNIT_ALL_TESTS depends on KUNIT default KUNIT_ALL_TESTS help Enable this option to test RTC library functions.
If unsure, say N.
config CONTEXT_TRACKING bool
config CONTEXT_TRACKING_IDLE bool select CONTEXT_TRACKING help Tracks idle state on behalf of RCU.
if GENERIC_CLOCKEVENTS menu "Timers subsystem"
Core internal switch. Selected by NO_HZ_COMMON / HIGH_RES_TIMERS. This is
only related to the tick functionality. Oneshot clockevent devices
are supported independent of this.
config TICK_ONESHOT bool
config NO_HZ_COMMON bool select TICK_ONESHOT
choice prompt "Timer tick handling" default NO_HZ_IDLE if NO_HZ
config HZ_PERIODIC bool "Periodic timer ticks (constant rate, no dynticks)" help This option keeps the tick running periodically at a constant rate, even when the CPU doesn't need it.
config NO_HZ_IDLE bool "Idle dynticks system (tickless idle)" select NO_HZ_COMMON help This option enables a tickless idle system: timer interrupts will only trigger on an as-needed basis when the system is idle. This is usually interesting for energy saving.
Most of the time you want to say Y here.
config NO_HZ_FULL bool "Full dynticks system (tickless)" # NO_HZ_COMMON dependency # We need at least one periodic CPU for timekeeping depends on SMP depends on HAVE_CONTEXT_TRACKING_USER # VIRT_CPU_ACCOUNTING_GEN dependency depends on HAVE_VIRT_CPU_ACCOUNTING_GEN select NO_HZ_COMMON select RCU_NOCB_CPU select VIRT_CPU_ACCOUNTING_GEN select IRQ_WORK select CPU_ISOLATION help Adaptively try to shutdown the tick whenever possible, even when the CPU is running tasks. Typically this requires running a single task on the CPU. Chances for running tickless are maximized when the task mostly runs in userspace and has few kernel activity.
You need to fill up the nohz_full boot parameter with the
desired range of dynticks CPUs to use it. This is implemented at
the expense of some overhead in user <-> kernel transitions:
syscalls, exceptions and interrupts.
By default, without passing the nohz_full parameter, this behaves just
like NO_HZ_IDLE.
If you're a distro say Y.
endchoice
config CONTEXT_TRACKING_USER bool depends on HAVE_CONTEXT_TRACKING_USER select CONTEXT_TRACKING help Track transitions between kernel and user on behalf of RCU and tickless cputime accounting. The former case relies on context tracking to enter/exit RCU extended quiescent states.
config CONTEXT_TRACKING_USER_FORCE bool "Force user context tracking" depends on CONTEXT_TRACKING_USER default y if !NO_HZ_FULL help The major pre-requirement for full dynticks to work is to support the user context tracking subsystem. But there are also other dependencies to provide in order to make the full dynticks working.
This option stands for testing when an arch implements the
user context tracking backend but doesn't yet fulfill all the
requirements to make the full dynticks feature working.
Without the full dynticks, there is no way to test the support
for user context tracking and the subsystems that rely on it: RCU
userspace extended quiescent state and tickless cputime
accounting. This option copes with the absence of the full
dynticks subsystem by forcing the user context tracking on all
CPUs in the system.
Say Y only if you're working on the development of an
architecture backend for the user context tracking.
Say N otherwise, this option brings an overhead that you
don't want in production.
config NO_HZ bool "Old Idle dynticks config" help This is the old config entry that enables dynticks idle. We keep it around for a little while to enforce backward compatibility with older config files.
config HIGH_RES_TIMERS bool "High Resolution Timer Support" select TICK_ONESHOT help This option enables high resolution timer support. If your hardware is not capable then this option only increases the size of the kernel image.
config CLOCKSOURCE_WATCHDOG_MAX_SKEW_US int "Clocksource watchdog maximum allowable skew (in μs)" depends on CLOCKSOURCE_WATCHDOG range 50 1000 default 125 help Specify the maximum amount of allowable watchdog skew in microseconds before reporting the clocksource to be unstable. The default is based on a half-second clocksource watchdog interval and NTP's maximum frequency drift of 500 parts per million. If the clocksource is good enough for NTP, it is good enough for the clocksource watchdog!
endmenu endif