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![]() Placing timers at enqueue time on a target CPU based on dubious heuristics does not make any sense: 1) Most timer wheel timers are canceled or rearmed before they expire. 2) The heuristics to predict which CPU will be busy when the timer expires are wrong by definition. So placing the timers at enqueue wastes precious cycles. The proper solution to this problem is to always queue the timers on the local CPU and allow the non pinned timers to be pulled onto a busy CPU at expiry time. Therefore split the timer storage into local pinned and global timers: Local pinned timers are always expired on the CPU on which they have been queued. Global timers can be expired on any CPU. As long as a CPU is busy it expires both local and global timers. When a CPU goes idle it arms for the first expiring local timer. If the first expiring pinned (local) timer is before the first expiring movable timer, then no action is required because the CPU will wake up before the first movable timer expires. If the first expiring movable timer is before the first expiring pinned (local) timer, then this timer is queued into an idle timerqueue and eventually expired by another active CPU. To avoid global locking the timerqueues are implemented as a hierarchy. The lowest level of the hierarchy holds the CPUs. The CPUs are associated to groups of 8, which are separated per node. If more than one CPU group exist, then a second level in the hierarchy collects the groups. Depending on the size of the system more than 2 levels are required. Each group has a "migrator" which checks the timerqueue during the tick for remote expirable timers. If the last CPU in a group goes idle it reports the first expiring event in the group up to the next group(s) in the hierarchy. If the last CPU goes idle it arms its timer for the first system wide expiring timer to ensure that no timer event is missed. Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Reviewed-by: Frederic Weisbecker <frederic@kernel.org> Link: https://lore.kernel.org/r/20240222103710.32582-1-anna-maria@linutronix.de |
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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 |