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Commit 8d165db1 authored by Anton Blanchard's avatar Anton Blanchard Committed by Benjamin Herrenschmidt
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powerpc: Improve decrementer accuracy 



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: default avatarAnton Blanchard <anton@samba.org>
Signed-off-by: default avatarBenjamin Herrenschmidt <benh@kernel.crashing.org>
parent 9aa4e7b1

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 =