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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>
This commit is contained in:
Adrian Hunter 2015-08-21 12:05:18 +03:00 committed by Ingo Molnar
parent 216dcaf290
commit b20112edea
1 changed files with 11 additions and 13 deletions
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24
arch/x86/kernel/tsc.c
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@ -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);