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| author | Linus Torvalds <torvalds@linux-foundation.org> | 2011-12-05 16:50:24 -0800 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2011-12-05 16:50:24 -0800 |
| commit | 7125faceabe43067293d0c9e2ef7154ecea51721 (patch) | |
| tree | 9de559e9280843d02d9a70976fbad23428161189 /arch | |
| parent | 35337c834124d2893b7fe4ba683c7639e6c37e0c (diff) | |
| parent | 4cecf6d401a01d054afc1e5f605bcbfe553cb9b9 (diff) | |
Merge branch 'sched-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
* 'sched-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
sched, x86: Avoid unnecessary overflow in sched_clock
sched: Fix buglet in return_cfs_rq_runtime()
sched: Avoid SMT siblings in select_idle_sibling() if possible
sched: Set the command name of the idle tasks in SMP kernels
sched, rt: Provide means of disabling cross-cpu bandwidth sharing
sched: Document wait_for_completion_*() return values
sched_fair: Fix a typo in the comment describing update_sd_lb_stats
sched: Add a comment to effective_load() since it's a pain
Diffstat (limited to 'arch')
| -rw-r--r-- | arch/x86/include/asm/timer.h | 23 |
1 files changed, 22 insertions, 1 deletions
diff --git a/arch/x86/include/asm/timer.h b/arch/x86/include/asm/timer.h index fa7b9176b76..431793e5d48 100644 --- a/arch/x86/include/asm/timer.h +++ b/arch/x86/include/asm/timer.h @@ -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; } |