When TSC is stable perf/sched clock is based on it.
However the conversion from cycles to nanoseconds
is not as accurate as it could be. Because
CYC2NS_SCALE_FACTOR is 10, the accuracy is +/- 1/2048
The change is to calculate the maximum shift that
results in a multiplier that is still a 32-bit number.
For example all frequencies over 1 GHz will have
a shift of 32, making the accuracy of the conversion
+/- 1/(2^33)
Signed-off-by: Adrian Hunter <adrian.hun...@intel.com>
---
arch/x86/kernel/tsc.c | 33 ++++++++++++++++++++-------------
1 file changed, 20 insertions(+), 13 deletions(-)
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 7437b41f6a47..e7085bcfb06b 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -167,21 +167,21 @@ static void cyc2ns_write_end(int cpu, struct cyc2ns_data
*data)
* ns = cycles * cyc2ns_scale / SC
*
* And since SC is a constant power of two, we can convert the div
- * into a shift.
+ * into a shift. The larger SC is, the more accurate the conversion, but
+ * cyc2ns_scale needs to be a 32-bit value so that 32-bit multiplication
+ * (64-bit result) can be used. So start by trying SC = 2^32, reducing
+ * until the criteria are met.
*
- * We can use khz divisor instead of mhz to keep a better precision, since
- * cyc2ns_scale is limited to 10^6 * 2^10, which fits in 32 bits.
+ * We can use khz divisor instead of mhz to keep a better precision.
* (mathieu.desnoy...@polymtl.ca)
*
* -johns...@us.ibm.com "math is hard, lets go shopping!"
*/
-#define CYC2NS_SCALE_FACTOR 10 /* 2^10, carefully chosen */
-
static void cyc2ns_data_init(struct cyc2ns_data *data)
{
data->cyc2ns_mul = 0;
- data->cyc2ns_shift = CYC2NS_SCALE_FACTOR;
+ data->cyc2ns_shift = 0;
data->cyc2ns_offset = 0;
data->__count = 0;
}
@@ -215,14 +215,14 @@ static inline unsigned long long cycles_2_ns(unsigned
long long cyc)
if (likely(data == tail)) {
ns = data->cyc2ns_offset;
- ns += mul_u64_u32_shr(cyc, data->cyc2ns_mul,
CYC2NS_SCALE_FACTOR);
+ ns += mul_u64_u32_shr(cyc, data->cyc2ns_mul,
data->cyc2ns_shift);
} else {
data->__count++;
barrier();
ns = data->cyc2ns_offset;
- ns += mul_u64_u32_shr(cyc, data->cyc2ns_mul,
CYC2NS_SCALE_FACTOR);
+ ns += mul_u64_u32_shr(cyc, data->cyc2ns_mul,
data->cyc2ns_shift);
barrier();
@@ -239,6 +239,8 @@ static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
unsigned long long tsc_now, ns_now;
struct cyc2ns_data *data;
unsigned long flags;
+ u64 mult;
+ u32 shft = 32;
local_irq_save(flags);
sched_clock_idle_sleep_event();
@@ -256,12 +258,17 @@ static void set_cyc2ns_scale(unsigned long cpu_khz, int
cpu)
* time function is continuous; see the comment near struct
* cyc2ns_data.
*/
- data->cyc2ns_mul =
- DIV_ROUND_CLOSEST(NSEC_PER_MSEC << CYC2NS_SCALE_FACTOR,
- cpu_khz);
- data->cyc2ns_shift = CYC2NS_SCALE_FACTOR;
+ mult = (u64)NSEC_PER_MSEC << 32;
+ mult += cpu_khz / 2;
+ do_div(mult, cpu_khz);
+ while (mult > U32_MAX) {
+ mult >>= 1;
+ shft -= 1;
+ }
+ data->cyc2ns_mul = mult;
+ data->cyc2ns_shift = shft;
data->cyc2ns_offset = ns_now -
- mul_u64_u32_shr(tsc_now, data->cyc2ns_mul, CYC2NS_SCALE_FACTOR);
+ mul_u64_u32_shr(tsc_now, data->cyc2ns_mul, data->cyc2ns_shift);
cyc2ns_write_end(cpu, data);
--
1.9.1
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