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The timekeeping duty is handed over from the outgoing CPU within stop machine. This works well if CONFIG_NO_HZ_COMMON=n or the tick is in high-res mode. However in low-res dynticks mode, the tick isn't cancelled until the clockevent is shut down, which can happen later. The tick may therefore fire again once IRQs are re-enabled on stop machine and until IRQs are disabled for good upon the last call to idle. That's so many opportunities for a timekeeper to go idle and the outgoing CPU to take over that duty. This is why tick_nohz_idle_stop_tick() is called one last time on idle if the CPU is seen offline: so that the timekeeping duty is handed over again in case the CPU has re-taken the duty. This means there are two timekeeping handovers on CPU down hotplug with different undocumented constraints and purposes: 1) A handover on stop machine for !dynticks || highres. All online CPUs are guaranteed to be non-idle and the timekeeping duty can be safely handed-over. The hrtimer tick is cancelled so it is guaranteed that in dynticks mode the outgoing CPU won't take again the duty. 2) A handover on last idle call for dynticks && lowres. Setting the duty to TICK_DO_TIMER_NONE makes sure that a CPU will take over the timekeeping. Prepare for consolidating the handover to a single place (the first one) with shutting down the low-res tick as well from tick_cancel_sched_timer() as well. This will simplify the handover and unify the tick cancellation between high-res and low-res. Signed-off-by: Frederic Weisbecker <frederic@kernel.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Reviewed-by: Thomas Gleixner <tglx@linutronix.de> Link: https://lore.kernel.org/r/20240225225508.11587-15-frederic@kernel.org |
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.. | ||
alarmtimer.c | ||
clockevents.c | ||
clocksource-wdtest.c | ||
clocksource.c | ||
hrtimer.c | ||
itimer.c | ||
jiffies.c | ||
Kconfig | ||
Makefile | ||
namespace.c | ||
ntp.c | ||
ntp_internal.h | ||
posix-clock.c | ||
posix-cpu-timers.c | ||
posix-stubs.c | ||
posix-timers.c | ||
posix-timers.h | ||
sched_clock.c | ||
test_udelay.c | ||
tick-broadcast-hrtimer.c | ||
tick-broadcast.c | ||
tick-common.c | ||
tick-internal.h | ||
tick-legacy.c | ||
tick-oneshot.c | ||
tick-sched.c | ||
tick-sched.h | ||
time.c | ||
time_test.c | ||
timeconst.bc | ||
timeconv.c | ||
timecounter.c | ||
timekeeping.c | ||
timekeeping.h | ||
timekeeping_debug.c | ||
timekeeping_internal.h | ||
timer.c | ||
timer_list.c | ||
timer_migration.c | ||
timer_migration.h | ||
vsyscall.c |