Re: [PATCH V2] perf: x86: Improve accuracy of perf/sched clock

[Adrian Hunter]

On 29/07/15 00:14, 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)
> 
> Signed-off-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>

Is this OK?

> ---
>  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.desnoyers@xxxxxxxxxx)
>   *
>   *                      -johnstul@xxxxxxxxxx "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);
>  
> 

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