The patch titled
common implementation of iterative div/mod
has been removed from the -mm tree. Its filename was
common-implementation-of-iterative-div-mod.patch
This patch was dropped because it had testing failures
The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/
------------------------------------------------------
Subject: common implementation of iterative div/mod
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
We have a few instances of the open-coded iterative div/mod loop, used when we
don't expect the dividend to be much bigger than the divisor. Unfortunately
modern gcc's have the tendency to strength "reduce" this into a full mod
operation, which isn't necessarily any faster, and even if it were, doesn't
exist if gcc implements it in libgcc.
The workaround is to put a dummy asm statement in the loop to prevent gcc from
performing the transformation.
This patch creates a single implementation of this loop, and uses it to
replace the open-coded versions I know about.
[akpm@xxxxxxxxxxxxxxxxxxxx: s/unsigned/u32/]
[akpm@xxxxxxxxxxxxxxxxxxxx: coding-style fix]
[akpm@xxxxxxxxxxxxxxxxxxxx: fix headers_check]
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: john stultz <johnstul@xxxxxxxxxx>
Cc: Segher Boessenkool <segher@xxxxxxxxxxxxxxxxxxx>
Cc: Christian Kujau <lists@xxxxxxxxxxxxxxx>
Cc: Robert Hancock <hancockr@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
arch/x86/xen/time.c | 13 +++----------
include/linux/math64.h | 2 ++
include/linux/time.h | 11 ++---------
lib/div64.c | 23 +++++++++++++++++++++++
4 files changed, 30 insertions(+), 19 deletions(-)
diff -puN arch/x86/xen/time.c~common-implementation-of-iterative-div-mod arch/x86/xen/time.c
--- a/arch/x86/xen/time.c~common-implementation-of-iterative-div-mod
+++ a/arch/x86/xen/time.c
@@ -12,6 +12,7 @@
#include <linux/clocksource.h>
#include <linux/clockchips.h>
#include <linux/kernel_stat.h>
+#include <linux/math64.h>
#include <asm/xen/hypervisor.h>
#include <asm/xen/hypercall.h>
@@ -150,11 +151,7 @@ static void do_stolen_accounting(void)
if (stolen < 0)
stolen = 0;
- ticks = 0;
- while (stolen >= NS_PER_TICK) {
- ticks++;
- stolen -= NS_PER_TICK;
- }
+ ticks = iter_div_u64_rem(stolen, NS_PER_TICK, &stolen);
__get_cpu_var(residual_stolen) = stolen;
account_steal_time(NULL, ticks);
@@ -166,11 +163,7 @@ static void do_stolen_accounting(void)
if (blocked < 0)
blocked = 0;
- ticks = 0;
- while (blocked >= NS_PER_TICK) {
- ticks++;
- blocked -= NS_PER_TICK;
- }
+ ticks = iter_div_u64_rem(blocked, NS_PER_TICK, &blocked);
__get_cpu_var(residual_blocked) = blocked;
account_steal_time(idle_task(smp_processor_id()), ticks);
}
diff -puN include/linux/math64.h~common-implementation-of-iterative-div-mod include/linux/math64.h
--- a/include/linux/math64.h~common-implementation-of-iterative-div-mod
+++ a/include/linux/math64.h
@@ -81,4 +81,6 @@ static inline s64 div_s64(s64 dividend,
}
#endif
+u32 iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder);
+
#endif /* _LINUX_MATH64_H */
diff -puN include/linux/time.h~common-implementation-of-iterative-div-mod include/linux/time.h
--- a/include/linux/time.h~common-implementation-of-iterative-div-mod
+++ a/include/linux/time.h
@@ -6,6 +6,7 @@
#ifdef __KERNEL__
# include <linux/cache.h>
# include <linux/seqlock.h>
+# include <linux/math64.h>
#endif
#ifndef _STRUCT_TIMESPEC
@@ -172,15 +173,7 @@ extern struct timeval ns_to_timeval(cons
*/
static inline void timespec_add_ns(struct timespec *a, u64 ns)
{
- ns += a->tv_nsec;
- while(unlikely(ns >= NSEC_PER_SEC)) {
- /* The following asm() prevents the compiler from
- * optimising this loop into a modulo operation. */
- asm("" : "+r"(ns));
-
- ns -= NSEC_PER_SEC;
- a->tv_sec++;
- }
+ a->tv_sec += iter_div_u64_rem(a->tv_nsec + ns, NSEC_PER_SEC, &ns);
a->tv_nsec = ns;
}
#endif /* __KERNEL__ */
diff -puN lib/div64.c~common-implementation-of-iterative-div-mod lib/div64.c
--- a/lib/div64.c~common-implementation-of-iterative-div-mod
+++ a/lib/div64.c
@@ -98,3 +98,26 @@ EXPORT_SYMBOL(div64_u64);
#endif
#endif /* BITS_PER_LONG == 32 */
+
+/*
+ * Iterative div/mod for use when dividend is not expected to be much
+ * bigger than divisor.
+ */
+u32 iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder)
+{
+ u32 ret = 0;
+
+ while (dividend >= divisor) {
+ /* The following asm() prevents the compiler from
+ optimising this loop into a modulo operation. */
+ asm("" : "+rm"(dividend));
+
+ dividend -= divisor;
+ ret++;
+ }
+
+ *remainder = dividend;
+
+ return ret;
+}
+EXPORT_SYMBOL(iter_div_u64_rem);
_
Patches currently in -mm which might be from jeremy@xxxxxxxx are
common-implementation-of-iterative-div-mod.patch
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