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Diffstat (limited to 'arch/x86/kernel/tsc.c')
-rw-r--r--arch/x86/kernel/tsc.c35
1 files changed, 22 insertions, 13 deletions
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index f86453801..6ada9d131 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -169,21 +169,20 @@ 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.
*
- * 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.desnoyers@polymtl.ca)
*
* -johnstul@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;
}
@@ -221,14 +220,14 @@ 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();
@@ -262,12 +261,22 @@ 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;
+ clocks_calc_mult_shift(&data->cyc2ns_mul, &data->cyc2ns_shift, cpu_khz,
+ NSEC_PER_MSEC, 0);
+
+ /*
+ * cyc2ns_shift is exported via arch_perf_update_userpage() where it is
+ * not expected to be greater than 31 due to the original published
+ * conversion algorithm shifting a 32-bit value (now specifies a 64-bit
+ * value) - refer perf_event_mmap_page documentation in perf_event.h.
+ */
+ if (data->cyc2ns_shift == 32) {
+ data->cyc2ns_shift = 31;
+ data->cyc2ns_mul >>= 1;
+ }
+
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);