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

Ron
2008-11-26T15:32:18Z

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@debian.org>

Thanks!