diff options
author | Anton Blanchard <anton@samba.org> | 2009-05-10 13:37:36 (GMT) |
---|---|---|
committer | Benjamin Herrenschmidt <benh@kernel.crashing.org> | 2009-05-21 05:44:24 (GMT) |
commit | 8d165db10772f238103c3e8f955c54145e5c07f3 (patch) | |
tree | 54f287e003df92f2d7b237a2382e4f68db9821cd /arch/powerpc/kernel | |
parent | 9aa4e7b1699d0fa197778da96de7e03fa2374f0a (diff) | |
download | linux-fsl-qoriq-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')
-rw-r--r-- | arch/powerpc/kernel/time.c | 21 |
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 = |