[PATCH] Optimise apply_slack() for faster execution
From: Chinmay V S
Date: Sun Dec 25 2011 - 16:06:32 EST
This patch simplifies the apply_slack() function in the kernel timer
subsystem. This is frequently called in mod_timer()/add_timer() and will
thus improve the performance of both the functions.
The existing logic used an intermediate-mask and a set of bitwise
operations using the mask to round-off the expires_limit variable. This
patch discards the intermediate-mask in favour of direct
shift-operations.
Signed-off-by: Chinmay V S <cvs268@xxxxxxxxx>
---
kernel/timer.c | 22 +++++++++-------------
1 files changed, 9 insertions(+), 13 deletions(-)
diff --git a/kernel/timer.c b/kernel/timer.c
index 9c3c62b..3fc1f48 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -739,37 +739,33 @@ EXPORT_SYMBOL(mod_timer_pending);
* Algorithm:
* 1) calculate the maximum (absolute) time
* 2) calculate the highest bit where the expires and new max are different
- * 3) use this bit to make a mask
- * 4) use the bitmask to round down the maximum time, so that all last
- * bits are zeros
+ * 3) use this bit to round down maximum time, so that all last bits are 0
*/
static inline
unsigned long apply_slack(struct timer_list *timer, unsigned long expires)
{
- unsigned long expires_limit, mask;
+ unsigned long slack = 0;
int bit;
if (timer->slack >= 0) {
- expires_limit = expires + timer->slack;
+ slack = timer->slack;
} else {
long delta = expires - jiffies;
if (delta < 256)
return expires;
- expires_limit = expires + delta / 256;
+ slack = delta/256;
}
- mask = expires ^ expires_limit;
- if (mask == 0)
- return expires;
- bit = find_last_bit(&mask, BITS_PER_LONG);
+ if (slack == 0)
+ return expires;
- mask = (1 << bit) - 1;
+ bit = find_last_bit(&slack, BITS_PER_LONG);
- expires_limit = expires_limit & ~(mask);
+ expires = (expires >> bit) << bit;
- return expires_limit;
+ return expires;
}
/**
--
1.7.0.4
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