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

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Message-Id: <20090508181559.4750800e.akpm@linux-foundation.org>
Date:	Fri, 8 May 2009 18:15:59 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ron <ron@...ian.org>
Cc:	mingo@...e.hu, a.p.zijlstra@...llo.nl, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fix for sched_clock() when using jiffies

On Sat, 9 May 2009 10:10:09 +0930 Ron <ron@...ian.org> wrote:

> On Fri, May 08, 2009 at 04:01:42PM -0700, Andrew Morton wrote:
> > On Sat, 9 May 2009 05:34:44 +0930
> > Ron <ron@...ian.org> wrote:
> > 
> > >  
> > > Account for the initial offset to the jiffy count.
> > > 
> > > Signed-off-by: Ron Lee <ron@...ian.org>
> > > 
> > > ---
> > >  kernel/sched_clock.c |    3 ++-
> > >  1 files changed, 2 insertions(+), 1 deletions(-)
> > > 
> > > diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c
> > > index a0b0852..a1567b1 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;
> > 
> > Why?  I assume that you encountered some problem which was fixed
> > by this patch.  What was that problem?
> 
> This was a resend of a patch that seemed to get a thumbs up, except
> for whitespace damage in what I originally sent, but which apparently
> then didn't get applied.  The original context to it was:
> 
>  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.

Removing printk_clock() always seemed a mildly wrong idea to me.

I'm sure we fixed this printk-timestamping ages and ages ago.  Maybe it
came back, or maybe it's somehow specific to your setup?

It's trivial to test, but I don't have the time to build and boot a
kernel right now :(

If the printk oddity is indeed being seen on all kernels then I'd
suggest that it be fixed right there in vprintk().  Because changing
sched_clock() adds unneeded overhead and partially defeats the intent
of INITIAL_JIFFIES, which is to catch code which is incorrectly
handling jiffy wrapping.

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