[tip:sched/urgent] sched, x86: Fix cpufreq + sched_clock() TSC scaling

[tip-bot for Peter Zijlstra]

Commit-ID:  702dd8c6624b353eb48dd20a0d36405cfe4f4f99
Gitweb:     http://git.kernel.org/tip/702dd8c6624b353eb48dd20a0d36405cfe4f4f99
Author:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Tue, 16 Jun 2009 12:34:17 -0700
Committer:  Ingo Molnar <mingo@xxxxxxx>
CommitDate: Tue, 16 Jun 2009 21:48:01 +0200

sched, x86: Fix cpufreq + sched_clock() TSC scaling

For freqency dependent TSCs we only scale the cycles, we do not account
for the discrepancy in absolute value. Add a constant factor so that:

  c1 + f1*cyc == c2 + f2*cyc

[ Impact: fix/reduce sched_clock() jumps across frequency changing events ]

Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
Chucked-on-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>


---
 arch/x86/include/asm/timer.h |    6 +++++-
 arch/x86/kernel/tsc.c        |    8 ++++++--
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/timer.h b/arch/x86/include/asm/timer.h
index bd37ed4..20ca9c4 100644
--- a/arch/x86/include/asm/timer.h
+++ b/arch/x86/include/asm/timer.h
@@ -45,12 +45,16 @@ extern int no_timer_check;
  */
 
 DECLARE_PER_CPU(unsigned long, cyc2ns);
+DECLARE_PER_CPU(unsigned long long, cyc2ns_offset);
 
 #define CYC2NS_SCALE_FACTOR 10 /* 2^10, carefully chosen */
 
 static inline unsigned long long __cycles_2_ns(unsigned long long cyc)
 {
-	return cyc * per_cpu(cyc2ns, smp_processor_id()) >> CYC2NS_SCALE_FACTOR;
+	int cpu = smp_processor_id();
+	unsigned long long ns = per_cpu(cyc2ns_offset, cpu);
+	ns += cyc * per_cpu(cyc2ns, cpu) >> CYC2NS_SCALE_FACTOR;
+	return ns;
 }
 
 static inline unsigned long long cycles_2_ns(unsigned long long cyc)
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 3e1c057..ef4dac5 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -589,22 +589,26 @@ EXPORT_SYMBOL(recalibrate_cpu_khz);
  */
 
 DEFINE_PER_CPU(unsigned long, cyc2ns);
+DEFINE_PER_CPU(unsigned long long, cyc2ns_offset);
 
 static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
 {
-	unsigned long long tsc_now, ns_now;
+	unsigned long long tsc_now, ns_now, *offset;
 	unsigned long flags, *scale;
 
 	local_irq_save(flags);
 	sched_clock_idle_sleep_event();
 
 	scale = &per_cpu(cyc2ns, cpu);
+	offset = &per_cpu(cyc2ns_offset, cpu);
 
 	rdtscll(tsc_now);
 	ns_now = __cycles_2_ns(tsc_now);
 
-	if (cpu_khz)
+	if (cpu_khz) {
 		*scale = (NSEC_PER_MSEC << CYC2NS_SCALE_FACTOR)/cpu_khz;
+		*offset = ns_now - (tsc_now * *scale >> CYC2NS_SCALE_FACTOR);
+	}
 
 	sched_clock_idle_wakeup_event(0);
 	local_irq_restore(flags);
--
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