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

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Message-ID: <20081126153152.GB28153@homer.shelbyville.oz>
Date:	Thu, 27 Nov 2008 02:01:52 +1030
From:	Ron <ron@...ian.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [patch] fix for sched_clock() when using jiffies

On Wed, Nov 26, 2008 at 04:16:38PM +0100, Peter Zijlstra wrote:
> On Thu, 2008-11-27 at 01:36 +1030, Ron wrote:
> > 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.
> > 
> 
> Looks good, except I suspect this line will now be longer than 80
> characters and you forgot to provide your signed-off-by line.

diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c
index 8178724..1ce2e53 100644
--- a/kernel/sched_clock.c
+++ b/kernel/sched_clock.c
@@ -37,7 +37,8 @@
  */
 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);
 }
 
 static __read_mostly int sched_clock_running;


 Signed-off-by: Ron Lee <ron@...ian.org>


Thanks!


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