linux-kernel - Re: [GIT PULL] cputime patch for 2.6.30-rc6

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Message-ID: <20090519104900.12e1f80c@skybase>
Date:	Tue, 19 May 2009 10:49:00 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Michael Abbott <michael@...neidae.co.uk>,
	Jan Engelhardt <jengelh@...ozas.de>
Subject: Re: [GIT PULL] cputime patch for 2.6.30-rc6

On Mon, 18 May 2009 17:24:03 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Mon, 2009-05-18 at 16:09 +0200, Martin Schwidefsky wrote:
> > Hi Linus,
> > 
> > please pull from 'cputime' branch of
> > 
> > 	git://git390.marist.edu/pub/scm/linux-2.6.git cputime
> > 
> > to receive the following updates:
> > 
> > Michael Abbott (1):
> >       Fix idle time field in /proc/uptime
> > 
> >  fs/proc/uptime.c |    8 +++++++-
> >  1 files changed, 7 insertions(+), 1 deletions(-)
> > 
> > diff --git a/fs/proc/uptime.c b/fs/proc/uptime.c
> > index 0c10a0b..c0ac0d7 100644
> > --- a/fs/proc/uptime.c
> > +++ b/fs/proc/uptime.c
> > @@ -4,13 +4,19 @@
> >  #include <linux/sched.h>
> >  #include <linux/seq_file.h>
> >  #include <linux/time.h>
> > +#include <linux/kernel_stat.h>
> >  #include <asm/cputime.h>
> >  
> >  static int uptime_proc_show(struct seq_file *m, void *v)
> >  {
> >  	struct timespec uptime;
> >  	struct timespec idle;
> > -	cputime_t idletime = cputime_add(init_task.utime, init_task.stime);
> > +	int len, i;
> > +	cputime_t idletime = 0;
> 
> cputime_zero, I guess..

Yes, this needs to be cputime_zero.
 
> > +	for_each_possible_cpu(i)
> > +		idletime = cputime64_add(idletime, kstat_cpu(i).cpustat.idle);
> > +	idletime = cputime64_to_clock_t(idletime);
> >  
> >  	do_posix_clock_monotonic_gettime(&uptime);
> >  	monotonic_to_bootbased(&uptime);
> 
> This is a world readable proc file, adding a for_each_possible_cpu() in
> there scares me a little (this wouldn't be the first and only such case
> though).
> 
> Suppose you have lots of cpus, and all those cpus are dirtying those
> cachelines (who's updating idle time when they're idle?), then this loop
> can cause a massive cacheline bounce fest.
> 
> Then think about userspace doing: 
>   while :; do cat /proc/uptime > /dev/null; done

Well, don't do stupid things like that. That falls into the same
category as programs calling gettimeofday all the time.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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