x86/tsc: Fix sched_clock() sync

While looking through the code I noticed that we initialize the cyc2ns
fields with a different cycle value for each CPU, resulting in a
slightly different 0 point for each CPU.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 23ce8b9a5d1f65edf9cf4a2201cb69ee395e232a..dda964c6387d7c99f1087891097554e2f9883c53 100644 (file)
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -137,9 +137,9 @@ static inline unsigned long long cycles_2_ns(unsigned long long cyc)
        return ns;
 }
 
-static void set_cyc2ns_scale(unsigned long khz, int cpu)
+static void __set_cyc2ns_scale(unsigned long khz, int cpu, unsigned long long tsc_now)
 {
-       unsigned long long tsc_now, ns_now;
+       unsigned long long ns_now;
        struct cyc2ns_data data;
        struct cyc2ns *c2n;
        unsigned long flags;
@@ -150,7 +150,6 @@ static void set_cyc2ns_scale(unsigned long khz, int cpu)
        if (!khz)
                goto done;
 
-       tsc_now = rdtsc();
        ns_now = cycles_2_ns(tsc_now);
 
        /*
@@ -186,6 +185,12 @@ done:
        sched_clock_idle_wakeup_event(0);
        local_irq_restore(flags);
 }
+
+static void set_cyc2ns_scale(unsigned long khz, int cpu)
+{
+       __set_cyc2ns_scale(khz, cpu, rdtsc());
+}
+
 /*
  * Scheduler clock - returns current time in nanosec units.
  */
@@ -1251,7 +1256,7 @@ device_initcall(init_tsc_clocksource);
 
 void __init tsc_init(void)
 {
-       u64 lpj;
+       u64 lpj, cyc;
        int cpu;
 
        if (!boot_cpu_has(X86_FEATURE_TSC)) {
@@ -1291,9 +1296,10 @@ void __init tsc_init(void)
         * speed as the bootup CPU. (cpufreq notifiers will fix this
         * up if their speed diverges)
         */
+       cyc = rdtsc();
        for_each_possible_cpu(cpu) {
                cyc2ns_init(cpu);
-               set_cyc2ns_scale(tsc_khz, cpu);
+               __set_cyc2ns_scale(tsc_khz, cpu, cyc);
        }
 
        if (tsc_disabled > 0)