On 21/08/15 12:05, Adrian Hunter wrote:
> 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). That is achieved by using the
> 'clocks_calc_mult_shift()' function.
>
> Signed-off-by: Adrian Hunter <adrian.hun...@intel.com>
Ping?
> ---
>
>
> Changes in V3:
>
> Use clocks_calc_mult_shift() instead of open-coded
> calculation
>
> Changes in V2:
>
> Remove definition of CYC2NS_SCALE_FACTOR
> Amend comments
>
>
> arch/x86/kernel/tsc.c | 24 +++++++++++-------------
> 1 file changed, 11 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index c8d52cb4cb6e..39ec3d07affd 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -167,21 +167,20 @@ 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.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 +214,14 @@ 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();
>
> @@ -256,12 +255,11 @@ 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);
> +
> 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);
>
>
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