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authorLinus Torvalds <torvalds@linux-foundation.org>2011-12-05 16:50:24 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2011-12-05 16:50:24 -0800
commit7125faceabe43067293d0c9e2ef7154ecea51721 (patch)
tree9de559e9280843d02d9a70976fbad23428161189 /arch
parent35337c834124d2893b7fe4ba683c7639e6c37e0c (diff)
parent4cecf6d401a01d054afc1e5f605bcbfe553cb9b9 (diff)
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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.h23
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;
}