linux-kernel - [patch] fix for sched_clock() when using jiffies

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Date:	Thu, 27 Nov 2008 01:36:45 +1030
From:	Ron <ron@...ian.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: [patch] fix for sched_clock() when using jiffies


Hi,

I'm in the process of updating a port for an ARM based chip we've been
working on, from 2.6.22-rc4'ish to the current HEAD of Linus' tree, and
I started seeing the following:

[    0.000000] PID hash table entries: 512 (order: 9, 2048 bytes)
[42949372.970000] Dentry cache hash table entries: 16384 (order: 4, 65536 bytes)

The reason appears to be that printk_clock() has been replaced with a
call to cpu_clock, which in our case currently falls back to the default
(weak) implementation of sched_clock() that uses jiffies -- but doesn't
account for the initial offset of the jiffy count.  The following simple
patch fixes it for me, in line with what printk_clock used to do.

Cheers,
Ron

diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c
index 8178724..d76814e 100644
--- a/kernel/sched_clock.c
+++ b/kernel/sched_clock.c
@@ -37,7 +37,7 @@
  */
 unsigned long long __attribute__((weak)) sched_clock(void)
 {
-       return (unsigned long long)jiffies * (NSEC_PER_SEC / HZ);
+       return (unsigned long long)(jiffies - INITIAL_JIFFIES) * (NSEC_PER_SEC / HZ);
 }
 

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