Frederic Weisbecker 53e87e3cdc timers/nohz: Last resort update jiffies on nohz_full IRQ entry ... When at least one CPU runs in nohz_full mode, a dedicated timekeeper CPU is guaranteed to stay online and to never stop its tick. Meanwhile on some rare case, the dedicated timekeeper may be running with interrupts disabled for a while, such as in stop_machine. If jiffies stop being updated, a nohz_full CPU may end up endlessly programming the next tick in the past, taking the last jiffies update monotonic timestamp as a stale base, resulting in an tick storm. Here is a scenario where it matters: 0) CPU 0 is the timekeeper and CPU 1 a nohz_full CPU. 1) A stop machine callback is queued to execute somewhere. 2) CPU 0 reaches MULTI_STOP_DISABLE_IRQ while CPU 1 is still in MULTI_STOP_PREPARE. Hence CPU 0 can't do its timekeeping duty. CPU 1 can still take IRQs. 3) CPU 1 receives an IRQ which queues a timer callback one jiffy forward. 4) On IRQ exit, CPU 1 schedules the tick one jiffy forward, taking last_jiffies_update as a base. But last_jiffies_update hasn't been updated for 2 jiffies since the timekeeper has interrupts disabled. 5) clockevents_program_event(), which relies on ktime_get(), observes that the expiration is in the past and therefore programs the min delta event on the clock. 6) The tick fires immediately, goto 3) 7) Tick storm, the nohz_full CPU is drown and takes ages to reach MULTI_STOP_DISABLE_IRQ, which is the only way out of this situation. Solve this with unconditionally updating jiffies if the value is stale on nohz_full IRQ entry. IRQs and other disturbances are expected to be rare enough on nohz_full for the unconditional call to ktime_get() to actually matter. Reported-by: Paul E. McKenney <paulmck@kernel.org> Signed-off-by: Frederic Weisbecker <frederic@kernel.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Tested-by: Paul E. McKenney <paulmck@kernel.org> Link: https://lore.kernel.org/r/20211026141055.57358-2-frederic@kernel.org		2021-12-02 15:07:22 +01:00
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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-cpu-timers: Clear task::posix_cputimers_work in copy_process()	2021-11-02 12:52:17 +01:00
posix-stubs.c
posix-timers.c	Merge branch 'akpm' (patches from Andrew)	2021-09-03 10:08:28 -07:00
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	timers/nohz: Last resort update jiffies on nohz_full IRQ entry	2021-12-02 15:07:22 +01:00
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
vsyscall.c