[PATCH RFC: kvm tsc virtualization 13/20] Combine observed TSC deviation into moving average

Zachary Amsden
2009-12-15T04:10:18Z

arch/x86/kvm/x86.c | 14 +++++++++++---
1 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c66dede..8bd362b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -914,6 +914,7 @@ static void sync_tsc_helper(int measure_cpu, s64 *delta, atomic_t *ready)
/*
* Average and trim the samples of any outliers; we use > 2 x sigma
*/
+static u64 tsc_deviation;
static s64 average_samples(s64 *samples, unsigned num_samples)
{
unsigned i, j;
@@ -936,6 +937,11 @@ static s64 average_samples(s64 *samples, unsigned num_samples)
stddev = stddev / (num_samples - 1);
stddev = int_sqrt(stddev);

+ /* Accumulate deviation measurement into a moving average */
+ tsc_deviation = tsc_deviation ?
+ ((tsc_deviation << 4) - tsc_deviation + stddev) >> 4 :
+ stddev;
+
/* Throw out anything outside 2x stddev */
stddev <<= 1;
average = 0, j = 0;
@@ -950,10 +956,12 @@ static s64 average_samples(s64 *samples, unsigned num_samples)
}
if (j > 0)
average = average / j;
- else
+ else {
printk(KERN_ERR "kvm: TSC samples failed to converge!\n");
- pr_debug("%s: mean = %lld, stddev = %llu, average = %lld\n",
- __func__, mean, stddev, average);
+ average = mean;
+ }
+ pr_debug("%s: mean=%lld, stddev=%llu, average=%lld hdev=%llu\n",
+ __func__, mean, stddev, average, tsc_deviation);

return average;
}
--
1.6.5.2