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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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