x86: scale cyc_2_nsec according to CPU frequency

scale the sched_clock() cyc_2_nsec scaling factor according to
CPU frequency changes.

[ mingo@elte.hu: simplified it and fixed it for SMP. ]

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 47 insertions, 10 deletions

diff --git a/arch/x86/kernel/tsc_64.c b/arch/x86/kernel/tsc_64.c
index 9c70af45b42..32edd2c50e9 100644
--- a/arch/x86/kernel/tsc_64.c
+++ b/arch/x86/kernel/tsc_64.c
@@ -10,6 +10,7 @@
 
 #include <asm/hpet.h>
 #include <asm/timex.h>
+#include <asm/timer.h>
 
 static int notsc __initdata = 0;
 
@@ -18,16 +19,48 @@ EXPORT_SYMBOL(cpu_khz);
 unsigned int tsc_khz;
 EXPORT_SYMBOL(tsc_khz);
 
-static unsigned int cyc2ns_scale __read_mostly;
+/* Accelerators for sched_clock()
+ * convert from cycles(64bits) => nanoseconds (64bits)
+ *  basic equation:
+ *              ns = cycles / (freq / ns_per_sec)
+ *              ns = cycles * (ns_per_sec / freq)
+ *              ns = cycles * (10^9 / (cpu_khz * 10^3))
+ *              ns = cycles * (10^6 / cpu_khz)
+ *
+ *      Then we use scaling math (suggested by george@mvista.com) to get:
+ *              ns = cycles * (10^6 * SC / cpu_khz) / SC
+ *              ns = cycles * cyc2ns_scale / SC
+ *
+ *      And since SC is a constant power of two, we can convert the div
+ *  into a shift.
+ *
+ *  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.
+ *  (mathieu.desnoyers@polymtl.ca)
+ *
+ *                      -johnstul@us.ibm.com "math is hard, lets go shopping!"
+ */
+DEFINE_PER_CPU(unsigned long, cyc2ns);
 
-static inline void set_cyc2ns_scale(unsigned long khz)
+static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
 {
-        cyc2ns_scale = (NSEC_PER_MSEC << NS_SCALE) / khz;
-}
+        unsigned long flags, prev_scale, *scale;
+        unsigned long long tsc_now, ns_now;
 
-static unsigned long long cycles_2_ns(unsigned long long cyc)
-{
-        return (cyc * cyc2ns_scale) >> NS_SCALE;
+        local_irq_save(flags);
+        sched_clock_idle_sleep_event();
+
+        scale = &per_cpu(cyc2ns, cpu);
+
+        rdtscll(tsc_now);
+        ns_now = __cycles_2_ns(tsc_now);
+
+        prev_scale = *scale;
+        if (cpu_khz)
+                *scale = (NSEC_PER_MSEC << CYC2NS_SCALE_FACTOR)/cpu_khz;
+
+        sched_clock_idle_wakeup_event(0);
+        local_irq_restore(flags);
 }
 
 unsigned long long sched_clock(void)
@@ -100,7 +133,9 @@ static int time_cpufreq_notifier(struct notifier_block *nb, unsigned long val,
                         mark_tsc_unstable("cpufreq changes");
         }
 
-        set_cyc2ns_scale(tsc_khz_ref);
+        preempt_disable();
+        set_cyc2ns_scale(tsc_khz_ref, smp_processor_id());
+        preempt_enable();
 
         return 0;
 }
@@ -151,7 +186,7 @@ static unsigned long __init tsc_read_refs(unsigned long *pm,
 void __init tsc_calibrate(void)
 {
         unsigned long flags, tsc1, tsc2, tr1, tr2, pm1, pm2, hpet1, hpet2;
-        int hpet = is_hpet_enabled();
+        int hpet = is_hpet_enabled(), cpu;
 
         local_irq_save(flags);
 
@@ -206,7 +241,9 @@ void __init tsc_calibrate(void)
         }
 
         tsc_khz = tsc2 / tsc1;
-        set_cyc2ns_scale(tsc_khz);
+
+        for_each_possible_cpu(cpu)
+                set_cyc2ns_scale(tsc_khz, cpu);
 }
 
 /*