powerpc: Improve decrementer accuracy (8d165db1) · Commits · 戴 / test

I have been looking at sources of OS jitter and notice that after a long NO_HZ idle period we wakeup too early: relative time (us) event timer irq exit 999946.405 timer irq entry 4.835 timer irq exit 21.685 timer irq entry 3.540 timer (tick_sched_timer) entry Here we slept for just under a second then took a timer interrupt that did nothing. 21.685 us later we wake up again and do the work. We set a rather low shift value of 16 for the decrementer clockevent, which I think is causing this issue. On this box we have a 207MHz decrementer and see: clockevent: decrementer mult[3501] shift[16] cpu[0] For calculations of large intervals this mult/shift combination could be off by a significant amount. I notice the sparc code has a loop that iterates to find a mult/shift combination that maximises the shift value while keeping mult under 32bit. With the patch below we get: clockevent: decrementer mult[35015c20] shift[32] cpu[15] And we no longer see the spurious wakeups. Signed-off-by:

Anton Blanchard <anton@samba.org> Signed-off-by:

Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/time.c

+18 −3

Original line number	Diff line number	Diff line
		@@ -109,7 +109,7 @@ static void decrementer_set_mode(enum clock_event_mode mode,
		static struct clock_event_device decrementer_clockevent = {
		.name = "decrementer",
		.rating = 200,
		.shift = 16,
		.shift = 0, /* To be filled in */
		.mult = 0, /* To be filled in */
		.irq = 0,
		.set_next_event = decrementer_set_next_event,
		@@ -843,6 +843,22 @@ static void decrementer_set_mode(enum clock_event_mode mode,
		decrementer_set_next_event(DECREMENTER_MAX, dev);
		}

		static void __init setup_clockevent_multiplier(unsigned long hz)
		{
		u64 mult, shift = 32;

		while (1) {
		mult = div_sc(hz, NSEC_PER_SEC, shift);
		if (mult && (mult >> 32UL) == 0UL)
		break;

		shift--;
		}

		decrementer_clockevent.shift = shift;
		decrementer_clockevent.mult = mult;
		}

		static void register_decrementer_clockevent(int cpu)
		{
		struct clock_event_device *dec = &per_cpu(decrementers, cpu).event;
		@@ -860,8 +876,7 @@ static void __init init_decrementer_clockevent(void)
		{
		int cpu = smp_processor_id();

		decrementer_clockevent.mult = div_sc(ppc_tb_freq, NSEC_PER_SEC,
		decrementer_clockevent.shift);
		setup_clockevent_multiplier(ppc_tb_freq);
		decrementer_clockevent.max_delta_ns =
		clockevent_delta2ns(DECREMENTER_MAX, &decrementer_clockevent);
		decrementer_clockevent.min_delta_ns =

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