perf/x86: Improve accuracy of perf/sched clock
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> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Arnaldo Carvalho de Melo <acme@kernel.org> Cc: Arnaldo Carvalho de Melo <acme@redhat.com> Cc: Jiri Olsa <jolsa@redhat.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Stephane Eranian <eranian@google.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Vince Weaver <vincent.weaver@maine.edu> Link: http://lkml.kernel.org/r/1440147918-22250-1-git-send-email-adrian.hunter@intel.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
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@ -167,21 +167,20 @@ static void cyc2ns_write_end(int cpu, struct cyc2ns_data *data)
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* ns = cycles * cyc2ns_scale / SC
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*
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* And since SC is a constant power of two, we can convert the div
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* into a shift.
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* into a shift. The larger SC is, the more accurate the conversion, but
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* cyc2ns_scale needs to be a 32-bit value so that 32-bit multiplication
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* (64-bit result) can be used.
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*
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* We can use khz divisor instead of mhz to keep a better precision, since
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* cyc2ns_scale is limited to 10^6 * 2^10, which fits in 32 bits.
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* We can use khz divisor instead of mhz to keep a better precision.
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* (mathieu.desnoyers@polymtl.ca)
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*
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* -johnstul@us.ibm.com "math is hard, lets go shopping!"
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*/
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#define CYC2NS_SCALE_FACTOR 10 /* 2^10, carefully chosen */
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static void cyc2ns_data_init(struct cyc2ns_data *data)
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{
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data->cyc2ns_mul = 0;
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data->cyc2ns_shift = CYC2NS_SCALE_FACTOR;
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data->cyc2ns_shift = 0;
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data->cyc2ns_offset = 0;
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data->__count = 0;
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}
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@ -215,14 +214,14 @@ static inline unsigned long long cycles_2_ns(unsigned long long cyc)
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if (likely(data == tail)) {
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ns = data->cyc2ns_offset;
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ns += mul_u64_u32_shr(cyc, data->cyc2ns_mul, CYC2NS_SCALE_FACTOR);
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ns += mul_u64_u32_shr(cyc, data->cyc2ns_mul, data->cyc2ns_shift);
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} else {
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data->__count++;
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barrier();
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ns = data->cyc2ns_offset;
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ns += mul_u64_u32_shr(cyc, data->cyc2ns_mul, CYC2NS_SCALE_FACTOR);
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ns += mul_u64_u32_shr(cyc, data->cyc2ns_mul, data->cyc2ns_shift);
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barrier();
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@ -256,12 +255,11 @@ static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
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* time function is continuous; see the comment near struct
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* cyc2ns_data.
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*/
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data->cyc2ns_mul =
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DIV_ROUND_CLOSEST(NSEC_PER_MSEC << CYC2NS_SCALE_FACTOR,
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cpu_khz);
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data->cyc2ns_shift = CYC2NS_SCALE_FACTOR;
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clocks_calc_mult_shift(&data->cyc2ns_mul, &data->cyc2ns_shift, cpu_khz,
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NSEC_PER_MSEC, 0);
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data->cyc2ns_offset = ns_now -
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mul_u64_u32_shr(tsc_now, data->cyc2ns_mul, CYC2NS_SCALE_FACTOR);
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mul_u64_u32_shr(tsc_now, data->cyc2ns_mul, data->cyc2ns_shift);
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cyc2ns_write_end(cpu, data);
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