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

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Date:	Wed, 26 Nov 2008 16:16:38 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Ron <ron@...ian.org>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [patch] fix for sched_clock() when using jiffies

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..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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