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authorPeter Zijlstra <peterz@infradead.org>2009-06-16 12:34:17 -0700
committerIngo Molnar <mingo@elte.hu>2009-06-17 16:03:54 +0200
commit84599f8a59e77699f18f06948cea171a349a3f0f (patch)
tree9918eccc9bcc2a91eb0f5609f9ad69c59ac66bd0
parent03347e2592078a90df818670fddf97a33eec70fb (diff)
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. Our current formula is: time = cycles * mult (where mult is a function of the cpu-speed on variable tsc machines) Suppose our current cycle count is 10, and we have a multiplier of 5, then our time value would end up being 50. Now cpufreq comes along and changes the multiplier to say 3 or 7, which would result in our time being resp. 30 or 70. That means that we can observe random jumps in the time value due to frequency changes in both fwd and bwd direction. So what this patch does is change the formula to: time = cycles * frequency + offset And we calculate offset so that time_before == time_after, thereby ridding us of these jumps in time. [ Impact: fix/reduce sched_clock() jumps across frequency changing events ] Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> LKML-Reference: <new-submission> Signed-off-by: Ingo Molnar <mingo@elte.hu> Chucked-on-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
-rw-r--r--arch/x86/include/asm/timer.h6
-rw-r--r--arch/x86/kernel/tsc.c8
2 files changed, 11 insertions, 3 deletions
diff --git a/arch/x86/include/asm/timer.h b/arch/x86/include/asm/timer.h
index bd37ed444a2..20ca9c4d468 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 3e1c057e98f..ef4dac50143 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);