I had a sudden surge of inspiration and decided to follow up on this
one a bit:
>
> 5. Second core
>
> Disabling the second core makes the noise go away. This might be a subset of 1., as I've been told that a stopped core enters C1.
>
I have now found a very hacky workaround that is slightly better than
disabling C3 altogether; making C3 exclusive to one core at a time. It
seems to kill the noise and the system now spends 50% in C3, instead of
0%.
I read somewhere that the Intel Duo:s have an extra low power mode that
is entered if both cores are suspended at the same time (disabling the
shared cache or something like that I guess). So perhaps that mode is
the culprit. I'm hoping there's a way to disable just that feature. Do
any of you know a way? Venkatesh perhaps?
Here's the patch I'm currently running at least:
diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
index 78d77c5..9a859fb 100644
--- a/drivers/cpuidle/governors/menu.c
+++ b/drivers/cpuidle/governors/menu.c
@@ -16,6 +16,8 @@
#define BREAK_FUZZ 4 /* 4 us */
+static atomic_t cur_deep_cpu = ATOMIC_INIT(-1);
+
struct menu_device {
int last_state_idx;
@@ -50,6 +52,13 @@ static int menu_select(struct cpuidle_device *dev)
break;
if (s->exit_latency > pm_qos_requirement(PM_QOS_CPU_DMA_LATENCY))
break;
+
+ if (s->flags & CPUIDLE_FLAG_DEEP) {
+ rmb();
+ if (atomic_cmpxchg(&cur_deep_cpu, -1, dev->cpu) != -1)
+ break;
+ wmb();
+ }
}
data->last_state_idx = i - 1;
@@ -80,6 +89,11 @@ static void menu_reflect(struct cpuidle_device *dev)
if (!(target->flags & CPUIDLE_FLAG_TIME_VALID))
measured_us = USEC_PER_SEC / HZ;
+ if (atomic_read(&cur_deep_cpu) == dev->cpu) {
+ atomic_set(&cur_deep_cpu, -1);
+ wmb();
+ }
+
/* Predict time remaining until next break event */
if (measured_us + BREAK_FUZZ < data->expected_us - target->exit_latency) {
data->predicted_us = max(measured_us, data->last_measured_us);
--
-- Pierre Ossman
Linux kernel, MMC maintainer http://www.kernel.org
PulseAudio, core developer http://pulseaudio.org
rdesktop, core developer http://www.rdesktop.org
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