[PATCH] i386 and x86_64 TSC set_cyc2ns_scale imprecision
I just found out that some precision is unnecessarily lost in the arch/i386/kernel/timers/timer_tsc.c:set_cyc2ns_scale function. It uses a cpu_mhz parameter when it could use a cpu_khz. In the specific case of an Intel P4 running at 3001.171 Mhz, the truncation to 3001 Mhz leads to an imprecision of 19 microseconds per second : this is very sad for a timer with nearly nanosecond accuracy. Fix the x86_64 architecture too. Cc: george anzinger <george@mvista.com> Cc: john stultz <johnstul@us.ibm.com> Cc: Andi Kleen <ak@muc.de> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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@ -30,23 +30,28 @@ static seqlock_t monotonic_lock = SEQLOCK_UNLOCKED;
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* basic equation:
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* ns = cycles / (freq / ns_per_sec)
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* ns = cycles * (ns_per_sec / freq)
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* ns = cycles * (10^9 / (cpu_mhz * 10^6))
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* ns = cycles * (10^3 / cpu_mhz)
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* ns = cycles * (10^9 / (cpu_khz * 10^3))
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* ns = cycles * (10^6 / cpu_khz)
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*
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* Then we use scaling math (suggested by george@mvista.com) to get:
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* ns = cycles * (10^3 * SC / cpu_mhz) / SC
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* ns = cycles * (10^6 * SC / cpu_khz) / SC
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* ns = cycles * cyc2ns_scale / SC
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*
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* And since SC is a constant power of two, we can convert the div
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* into a shift.
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*
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* We can use khz divisor instead of mhz to keep a better percision, since
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* cyc2ns_scale is limited to 10^6 * 2^10, which fits in 32 bits.
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* (mathieu.desnoyers@polymtl.ca)
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*
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* -johnstul@us.ibm.com "math is hard, lets go shopping!"
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*/
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static unsigned long cyc2ns_scale;
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#define CYC2NS_SCALE_FACTOR 10 /* 2^10, carefully chosen */
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static inline void set_cyc2ns_scale(unsigned long cpu_mhz)
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static inline void set_cyc2ns_scale(unsigned long cpu_khz)
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{
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cyc2ns_scale = (1000 << CYC2NS_SCALE_FACTOR)/cpu_mhz;
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cyc2ns_scale = (1000000 << CYC2NS_SCALE_FACTOR)/cpu_khz;
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}
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static inline unsigned long long cycles_2_ns(unsigned long long cyc)
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@ -163,7 +168,7 @@ static int __init init_hpet(char* override)
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printk("Detected %u.%03u MHz processor.\n",
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cpu_khz / 1000, cpu_khz % 1000);
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}
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set_cyc2ns_scale(cpu_khz/1000);
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set_cyc2ns_scale(cpu_khz);
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}
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/* set this only when cpu_has_tsc */
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timer_hpet.read_timer = read_timer_tsc;
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@ -49,23 +49,28 @@ static seqlock_t monotonic_lock = SEQLOCK_UNLOCKED;
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* basic equation:
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* ns = cycles / (freq / ns_per_sec)
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* ns = cycles * (ns_per_sec / freq)
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* ns = cycles * (10^9 / (cpu_mhz * 10^6))
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* ns = cycles * (10^3 / cpu_mhz)
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* ns = cycles * (10^9 / (cpu_khz * 10^3))
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* ns = cycles * (10^6 / cpu_khz)
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*
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* Then we use scaling math (suggested by george@mvista.com) to get:
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* ns = cycles * (10^3 * SC / cpu_mhz) / SC
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* ns = cycles * (10^6 * SC / cpu_khz) / SC
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* ns = cycles * cyc2ns_scale / SC
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*
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* And since SC is a constant power of two, we can convert the div
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* into a shift.
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* into a shift.
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*
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* We can use khz divisor instead of mhz to keep a better percision, since
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* cyc2ns_scale is limited to 10^6 * 2^10, which fits in 32 bits.
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* (mathieu.desnoyers@polymtl.ca)
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*
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* -johnstul@us.ibm.com "math is hard, lets go shopping!"
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*/
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static unsigned long cyc2ns_scale;
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#define CYC2NS_SCALE_FACTOR 10 /* 2^10, carefully chosen */
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static inline void set_cyc2ns_scale(unsigned long cpu_mhz)
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static inline void set_cyc2ns_scale(unsigned long cpu_khz)
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{
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cyc2ns_scale = (1000 << CYC2NS_SCALE_FACTOR)/cpu_mhz;
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cyc2ns_scale = (1000000 << CYC2NS_SCALE_FACTOR)/cpu_khz;
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}
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static inline unsigned long long cycles_2_ns(unsigned long long cyc)
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@ -286,7 +291,7 @@ time_cpufreq_notifier(struct notifier_block *nb, unsigned long val,
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if (use_tsc) {
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if (!(freq->flags & CPUFREQ_CONST_LOOPS)) {
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fast_gettimeoffset_quotient = cpufreq_scale(fast_gettimeoffset_ref, freq->new, ref_freq);
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set_cyc2ns_scale(cpu_khz/1000);
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set_cyc2ns_scale(cpu_khz);
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}
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}
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#endif
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@ -536,7 +541,7 @@ static int __init init_tsc(char* override)
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printk("Detected %u.%03u MHz processor.\n",
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cpu_khz / 1000, cpu_khz % 1000);
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}
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set_cyc2ns_scale(cpu_khz/1000);
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set_cyc2ns_scale(cpu_khz);
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return 0;
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}
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}
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@ -481,9 +481,9 @@ static irqreturn_t timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
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static unsigned int cyc2ns_scale;
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#define CYC2NS_SCALE_FACTOR 10 /* 2^10, carefully chosen */
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static inline void set_cyc2ns_scale(unsigned long cpu_mhz)
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static inline void set_cyc2ns_scale(unsigned long cpu_khz)
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{
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cyc2ns_scale = (1000 << CYC2NS_SCALE_FACTOR)/cpu_mhz;
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cyc2ns_scale = (1000000 << CYC2NS_SCALE_FACTOR)/cpu_khz;
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}
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static inline unsigned long long cycles_2_ns(unsigned long long cyc)
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@ -655,7 +655,7 @@ static int time_cpufreq_notifier(struct notifier_block *nb, unsigned long val,
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vxtime.tsc_quot = (1000L << 32) / cpu_khz;
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}
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set_cyc2ns_scale(cpu_khz_ref / 1000);
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set_cyc2ns_scale(cpu_khz_ref);
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return 0;
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}
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@ -939,7 +939,7 @@ void __init time_init(void)
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rdtscll_sync(&vxtime.last_tsc);
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setup_irq(0, &irq0);
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set_cyc2ns_scale(cpu_khz / 1000);
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set_cyc2ns_scale(cpu_khz);
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#ifndef CONFIG_SMP
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time_init_gtod();
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