linux-kernel - [PATCH] [timer] Optimise apply

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Message-Id: <1328284850-27743-1-git-send-email-chinmay.v.s@pathpartnertech.com>
Date:	Fri,  3 Feb 2012 21:30:50 +0530
From:	Chinmay V S <chinmay.v.s@...hpartnertech.com>
To:	linux-kernel@...r.kernel.org
Cc:	cvs268@...il.com, tglx@...utronix.de, sebastian@...akpoint.cc,
	arjan@...ux.intel.com, jeff.chua.linux@...il.com,
	Chinmay V S <chinmay.v.s@...hpartnertech.com>
Subject: [PATCH] [timer] Optimise apply_slack() for size and speed.

Updated the patch for improved readability.
How about the following patch?


To apply proper slack, the original algorithm used to prepare a mask and
then apply the mask to obtain the appropriately rounded-off absolute
time the timer expires.

This patch modifies the masking logic to a bit-shift logic, therby
reducing the complexity and number of operations. Thus obtaining a minor
speed-up.

Signed-off-by: Chinmay V S <chinmay.v.s@...hpartnertech.com>
---
 kernel/timer.c |    9 +++------
 1 files changed, 3 insertions(+), 6 deletions(-)

diff --git a/kernel/timer.c b/kernel/timer.c
index a297ffc..eb4b708 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -785,9 +785,7 @@ 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) round down the maximum time, so that all the lower bits are zeros
  */
 static inline
 unsigned long apply_slack(struct timer_list *timer, unsigned long expires)
@@ -811,9 +809,8 @@ unsigned long apply_slack(struct timer_list *timer, unsigned long expires)
 
 	bit = find_last_bit(&mask, BITS_PER_LONG);
 
-	mask = (1 << bit) - 1;
-
-	expires_limit = expires_limit & ~(mask);
+	/* Round down by zero-ing the lower bits */
+	expires_limit = (expires_limit >> bit) << bit;
 
 	return expires_limit;
 }
-- 
1.7.5.4

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