perf/x86: Improve accuracy of perf/sched clock

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 <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vince Weaver <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>