sched, x86: Avoid unnecessary overflow in sched_clock (4cecf6d4) · Commits · 戴 / test

(Added the missing signed-off-by line) In hundreds of days, the __cycles_2_ns calculation in sched_clock has an overflow. cyc * per_cpu(cyc2ns, cpu) exceeds 64 bits, causing the final value to become zero. We can solve this without losing any precision. We can decompose TSC into quotient and remainder of division by the scale factor, and then use this to convert TSC into nanoseconds. Signed-off-by:

Salman Qazi <sqazi@google.com> Acked-by:

John Stultz <johnstul@us.ibm.com> Reviewed-by:

Paul Turner <pjt@google.com> Cc: stable@kernel.org Signed-off-by:

Peter Zijlstra <a.p.zijlstra@chello.nl> Link: http://lkml.kernel.org/r/20111115221121.7262.88871.stgit@dungbeetle.mtv.corp.google.com Signed-off-by:

Ingo Molnar <mingo@elte.hu>

arch/x86/include/asm/timer.h

+22 −1

Original line number	Diff line number	Diff line
		@@ -32,6 +32,22 @@ extern int no_timer_check;
		* (mathieu.desnoyers@polymtl.ca)
		*
		* -johnstul@us.ibm.com "math is hard, lets go shopping!"
		*
		* In:
		*
		* ns = cycles * cyc2ns_scale / SC
		*
		* Although we may still have enough bits to store the value of ns,
		* in some cases, we may not have enough bits to store cycles * cyc2ns_scale,
		* leading to an incorrect result.
		*
		* To avoid this, we can decompose 'cycles' into quotient and remainder
		* of division by SC. Then,
		*
		* ns = (quot * SC + rem) * cyc2ns_scale / SC
		* = quot * cyc2ns_scale + (rem * cyc2ns_scale) / SC
		*
		* - sqazi@google.com
		*/

		DECLARE_PER_CPU(unsigned long, cyc2ns);
		@@ -41,9 +57,14 @@ DECLARE_PER_CPU(unsigned long long, cyc2ns_offset);

		static inline unsigned long long __cycles_2_ns(unsigned long long cyc)
		{
		unsigned long long quot;
		unsigned long long rem;
		int cpu = smp_processor_id();
		unsigned long long ns = per_cpu(cyc2ns_offset, cpu);
		ns += cyc * per_cpu(cyc2ns, cpu) >> CYC2NS_SCALE_FACTOR;
		quot = (cyc >> CYC2NS_SCALE_FACTOR);
		rem = cyc & ((1ULL << CYC2NS_SCALE_FACTOR) - 1);
		ns += quot * per_cpu(cyc2ns, cpu) +
		((rem * per_cpu(cyc2ns, cpu)) >> CYC2NS_SCALE_FACTOR);
		return ns;
		}

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