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Commit b20112ed authored by Adrian Hunter's avatar Adrian Hunter Committed by Ingo Molnar
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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: default avatarAdrian Hunter <adrian.hunter@intel.com>
Signed-off-by: default avatarPeter 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: default avatarIngo Molnar <mingo@kernel.org>
parent 216dcaf2

arch/x86/kernel/tsc.c

+11 −13
Original line number Diff line number Diff line
@@ -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);