From 84599f8a59e77699f18f06948cea171a349a3f0f Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Tue, 16 Jun 2009 12:34:17 -0700 Subject: 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 Cc: Peter Zijlstra LKML-Reference: Signed-off-by: Ingo Molnar Chucked-on-by: Thomas Gleixner Signed-off-by: Andrew Morton --- arch/x86/include/asm/timer.h | 6 +++++- arch/x86/kernel/tsc.c | 8 ++++++-- 2 files changed, 11 insertions(+), 3 deletions(-) (limited to 'arch') 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); -- cgit v1.1