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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: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
This commit is contained in:
Anton Blanchard 2009-05-10 13:37:36 +00:00 committed by Benjamin Herrenschmidt
parent 9aa4e7b169
commit 8d165db107
1 changed files with 18 additions and 3 deletions
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21
arch/powerpc/kernel/time.c
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@ -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 =