linux-kernel - Re: [PATCH 16/23] mm: scalable bdi statistics counters.

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Date:	Fri, 17 Aug 2007 18:23:02 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Josef Sipek <jsipek@....cs.sunysb.edu>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	miklos@...redi.hu, akpm@...ux-foundation.org, neilb@...e.de,
	dgc@....com, tomoki.sekiyama.qu@...achi.com, nikita@...sterfs.com,
	trond.myklebust@....uio.no, yingchao.zhou@...il.com,
	richard@....demon.co.uk, torvalds@...ux-foundation.org
Subject: Re: [PATCH 16/23] mm: scalable bdi statistics counters.

On Fri, 2007-08-17 at 12:20 -0400, Josef Sipek wrote:
> On Thu, Aug 16, 2007 at 09:45:41AM +0200, Peter Zijlstra wrote:
> ....
> > Index: linux-2.6/include/linux/backing-dev.h
> > ===================================================================
> > --- linux-2.6.orig/include/linux/backing-dev.h
> > +++ linux-2.6/include/linux/backing-dev.h
> ....
> > @@ -24,6 +26,12 @@ enum bdi_state {
> >  
> >  typedef int (congested_fn)(void *, int);
> >  
> > +enum bdi_stat_item {
> > +	NR_BDI_STAT_ITEMS
> > +};
> 
> enum numbering starts at 0, so NR_BDI_STAT_ITEMS == 0
> 
> > +
> > +#define BDI_STAT_BATCH (8*(1+ilog2(nr_cpu_ids)))
> > +
> >  struct backing_dev_info {
> >  	unsigned long ra_pages;	/* max readahead in PAGE_CACHE_SIZE units */
> >  	unsigned long state;	/* Always use atomic bitops on this */
> > @@ -32,15 +40,86 @@ struct backing_dev_info {
> >  	void *congested_data;	/* Pointer to aux data for congested func */
> >  	void (*unplug_io_fn)(struct backing_dev_info *, struct page *);
> >  	void *unplug_io_data;
> > +
> > +	struct percpu_counter bdi_stat[NR_BDI_STAT_ITEMS];
> 
> So, this is a 0-element array.
> 
> >  };
> >  
> > -static inline int bdi_init(struct backing_dev_info *bdi)
> > +int bdi_init(struct backing_dev_info *bdi);
> > +void bdi_destroy(struct backing_dev_info *bdi);
> > +
> > +static inline void __add_bdi_stat(struct backing_dev_info *bdi,
> > +		enum bdi_stat_item item, s64 amount)
> >  {
> > -	return 0;
> > +	__percpu_counter_add(&bdi->bdi_stat[item], amount, BDI_STAT_BATCH);
> 
> Boom!
> 
> >  }

Quite so, but since there are no callers _yet_ it will not go boom :-)

This patch introduces the framework, patch 17 and 18 will introduce both
stat items and callers.

So it should all work out just fine.

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