linux-kernel - Re: [PATCH 16/17] fs: Convert nr

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Message-ID: <20100930061039.GX5665@dastard>
Date:	Thu, 30 Sep 2010 16:10:39 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 16/17] fs: Convert nr_inodes to a per-cpu counter

On Wed, Sep 29, 2010 at 09:53:22PM -0700, Andrew Morton wrote:
> On Wed, 29 Sep 2010 22:18:48 +1000 Dave Chinner <david@...morbit.com> wrote:
> 
> > From: Eric Dumazet <dada1@...mosbay.com>
> > 
> > The number of inodes allocated does not need to be tied to the
> > addition or removal of an inode to/from a list. If we are not tied
> > to a list lock, we could update the counters when inodes are
> > initialised or destroyed, but to do that we need to convert the
> > counters to be per-cpu (i.e. independent of a lock). This means that
> > we have the freedom to change the list/locking implementation
> > without needing to care about the counters.
> > 
> >
> > ...
> >
> > +int get_nr_inodes(void)
> > +{
> > +	int i;
> > +	int sum = 0;
> > +	for_each_possible_cpu(i)
> > +		sum += per_cpu(nr_inodes, i);
> > +	return sum < 0 ? 0 : sum;
> > +}
> 
> This reimplements percpu_counter_sum_positive(), rather poorly

I thought so - it was on my list of things to check when redoing
this patch. I'll fix that up appropritately.

> 
> If one never intends to use the approximate percpu_counter_read() then
> one could initialise the counter with a really large batch value, for a
> very small performance gain.
> 
> > +int get_nr_inodes_unused(void)
> > +{
> > +	return inodes_stat.nr_unused;
> > +}
> >
> > ...
> >
> > @@ -407,6 +407,8 @@ extern struct files_stat_struct files_stat;
> >  extern int get_max_files(void);
> >  extern int sysctl_nr_open;
> >  extern struct inodes_stat_t inodes_stat;
> > +extern int get_nr_inodes(void);
> > +extern int get_nr_inodes_unused(void);
> 
> These are pretty cruddy names.  Unfotunately we don't really have a vfs
> or "inode" subsystem name to prefix them with.

Will fix.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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