Messages in this thread | | | Date | Tue, 01 Sep 2015 11:36:38 +0300 | From | Adrian Hunter <> | Subject | Re: [PATCH V3] perf: x86: Improve accuracy of perf/sched clock |
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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.hunter@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.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; > } > @@ -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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