9993bc635d
When a machine boots up, the TSC generally gets reset. However, when kexec is used to boot into a kernel, the TSC value would be carried over from the previous kernel. The computation of cycns_offset in set_cyc2ns_scale is prone to an overflow, if the machine has been up more than 208 days prior to the kexec. The overflow happens when we multiply *scale, even though there is enough room to store the final answer. We fix this issue by decomposing tsc_now into the quotient and remainder of division by CYC2NS_SCALE_FACTOR and then performing the multiplication separately on the two components. Refactor code to share the calculation with the previous fix in __cycles_2_ns(). Signed-off-by: Salman Qazi <sqazi@google.com> Acked-by: John Stultz <john.stultz@linaro.org> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Paul Turner <pjt@google.com> Cc: john stultz <johnstul@us.ibm.com> Link: http://lkml.kernel.org/r/20120310004027.19291.88460.stgit@dungbeetle.mtv.corp.google.com Signed-off-by: Ingo Molnar <mingo@elte.hu>
80 lines
2.1 KiB
C
80 lines
2.1 KiB
C
#ifndef _ASM_X86_TIMER_H
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#define _ASM_X86_TIMER_H
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#include <linux/init.h>
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#include <linux/pm.h>
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#include <linux/percpu.h>
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#include <linux/interrupt.h>
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#define TICK_SIZE (tick_nsec / 1000)
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unsigned long long native_sched_clock(void);
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extern int recalibrate_cpu_khz(void);
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extern int no_timer_check;
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/* Accelerators for sched_clock()
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* convert from cycles(64bits) => nanoseconds (64bits)
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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_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 * (10^6 * 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 precision, 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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* In:
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*
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* ns = cycles * cyc2ns_scale / SC
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*
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* Although we may still have enough bits to store the value of ns,
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* in some cases, we may not have enough bits to store cycles * cyc2ns_scale,
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* leading to an incorrect result.
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*
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* To avoid this, we can decompose 'cycles' into quotient and remainder
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* of division by SC. Then,
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*
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* ns = (quot * SC + rem) * cyc2ns_scale / SC
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* = quot * cyc2ns_scale + (rem * cyc2ns_scale) / SC
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*
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* - sqazi@google.com
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*/
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DECLARE_PER_CPU(unsigned long, cyc2ns);
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DECLARE_PER_CPU(unsigned long long, cyc2ns_offset);
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#define CYC2NS_SCALE_FACTOR 10 /* 2^10, carefully chosen */
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static inline unsigned long long __cycles_2_ns(unsigned long long cyc)
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{
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int cpu = smp_processor_id();
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unsigned long long ns = per_cpu(cyc2ns_offset, cpu);
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ns += mult_frac(cyc, per_cpu(cyc2ns, cpu),
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(1UL << CYC2NS_SCALE_FACTOR));
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return ns;
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}
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static inline unsigned long long cycles_2_ns(unsigned long long cyc)
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{
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unsigned long long ns;
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unsigned long flags;
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local_irq_save(flags);
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ns = __cycles_2_ns(cyc);
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local_irq_restore(flags);
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return ns;
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}
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#endif /* _ASM_X86_TIMER_H */
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