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authorAnton Blanchard <anton@samba.org>2009-05-10 13:37:36 (GMT)
committerBenjamin Herrenschmidt <benh@kernel.crashing.org>2009-05-21 05:44:24 (GMT)
commit8d165db10772f238103c3e8f955c54145e5c07f3 (patch)
tree54f287e003df92f2d7b237a2382e4f68db9821cd /arch/powerpc/kernel/time.c
parent9aa4e7b1699d0fa197778da96de7e03fa2374f0a (diff)
downloadlinux-8d165db10772f238103c3e8f955c54145e5c07f3.tar.xz
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>
Diffstat (limited to 'arch/powerpc/kernel/time.c')
-rw-r--r--arch/powerpc/kernel/time.c21
1 files changed, 18 insertions, 3 deletions
diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index 48571ac..bee1443 100644
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -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 =