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 <an...@samba.org>
---
- I haven't tested if it does the right thing on 32bit yet
- Should we do something similar to the timebase? We use a 22 bit shift
there but time might drift if that isnt accurate enough.
Index: linux-2.6/arch/powerpc/kernel/time.c
===================================================================
--- linux-2.6.orig/arch/powerpc/kernel/time.c 2009-05-10 19:48:39.000000000
+1000
+++ linux-2.6/arch/powerpc/kernel/time.c 2009-05-11 09:36:25.000000000
+1000
@@ -110,7 +110,7 @@
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,
@@ -852,6 +852,22 @@
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;
@@ -869,8 +885,7 @@
{
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 =
_______________________________________________
Linuxppc-dev mailing list
Linuxppc-dev@ozlabs.org
https://ozlabs.org/mailman/listinfo/linuxppc-dev
