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SubjectRe: [PATCH 16/23] mm: scalable bdi statistics counters.
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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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