From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
Subject: Re: [PATCH 4/6] fs: Introduce a per_cpu nr_inodes
Date: Thu, 27 Nov 2008 10:39:31 +0100
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Cc: Ingo Molnar <mingo-X9Un+BFzKDI@public.gmane.org>, David Miller <davem-fT/PcQaiUtIeIZ0/mPfg9Q@public.gmane.org>,
	"Rafael J. Wysocki" <rjw-KKrjLPT3xs0@public.gmane.org>, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	kernel-testers-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Mike Galbraith <efault-Mmb7MZpHnFY@public.gmane.org>,
	Linux Netdev List <netdev-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	Christoph Lameter <cl-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>,
	Christoph Hellwig <hch-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>, travis <travis-sJ/iWh9BUns@public.gmane.org>
To: Eric Dumazet <dada1-fPLkHRcR87vqlBn2x/YWAg@public.gmane.org>
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On Thu, 2008-11-27 at 10:33 +0100, Peter Zijlstra wrote:
> On Thu, 2008-11-27 at 00:32 +0100, Eric Dumazet wrote:
> > Avoids cache line ping pongs between cpus and prepare next patch,
> > because updates of nr_inodes metric dont need inode_lock anymore.
> > 
> > (socket8 bench result : 25s to 20.5s)
> > 
> > Signed-off-by: Eric Dumazet <dada1-fPLkHRcR87vqlBn2x/YWAg@public.gmane.org>
> > ---
> 
> > @@ -96,9 +96,40 @@ static DEFINE_MUTEX(iprune_mutex);
> >   * Statistics gathering..
> >   */
> >  struct inodes_stat_t inodes_stat;
> > +static DEFINE_PER_CPU(int, nr_inodes);
> >  
> >  static struct kmem_cache * inode_cachep __read_mostly;
> >  
> > +int get_nr_inodes(void)
> > +{
> > +	int cpu;
> > +	int counter = 0;
> > +
> > +	for_each_possible_cpu(cpu)
> > +	    counter += per_cpu(nr_inodes, cpu);
> > +	if (counter < 0)
> > +		counter = 0;
> > +	return counter;
> > +}
> 
> It would be good to get a cpu hotplug handler here and move to
> for_each_online_cpu(). People are wanting distro's to be build with
> NR_CPUS=4096.

Also, this trade-off between global vs per_cpu only works if
get_nr_inodes() is called significantly less than nr_inodes is changed.

With it being called from writeback that might not be true for all
workloads. One thing you can do about it is use the regular per-cpu
counter stuff, which allows you to do an approximation of the global
number (it also does all the hotplug stuff for you already).