On Sat, 2007-04-21 at 02:55 -0700, Andrew Morton wrote:
> On Fri, 20 Apr 2007 17:52:02 +0200 Peter Zijlstra <[email protected]> wrote:
>
> > Count per BDI writeback pages.
> >
> > Signed-off-by: Peter Zijlstra <[email protected]>
> > ---
> > include/linux/backing-dev.h | 1 +
> > mm/page-writeback.c | 12 ++++++++++--
> > 2 files changed, 11 insertions(+), 2 deletions(-)
> >
> > Index: linux-2.6/mm/page-writeback.c
> > ===================================================================
> > --- linux-2.6.orig/mm/page-writeback.c 2007-04-20 15:27:28.000000000 +0200
> > +++ linux-2.6/mm/page-writeback.c 2007-04-20 15:28:10.000000000 +0200
> > @@ -979,14 +979,18 @@ int test_clear_page_writeback(struct pag
> > int ret;
> >
> > if (mapping) {
> > + struct backing_dev_info *bdi = mapping->backing_dev_info;
> > unsigned long flags;
> >
> > write_lock_irqsave(&mapping->tree_lock, flags);
> > ret = TestClearPageWriteback(page);
> > - if (ret)
> > + if (ret) {
> > radix_tree_tag_clear(&mapping->page_tree,
> > page_index(page),
> > PAGECACHE_TAG_WRITEBACK);
> > + if (bdi_cap_writeback_dirty(bdi))
> > + __dec_bdi_stat(bdi, BDI_WRITEBACK);
>
> Why do we test bdi_cap_writeback_dirty() here?
>
> If we remove that test, we end up accumulating statistics for
> non-writebackable backing devs, but does that matter?
It would not, had I not cheated:
+void bdi_init(struct backing_dev_info *bdi)
+{
+ int i;
+
+ if (!(bdi_cap_writeback_dirty(bdi) || bdi_cap_account_dirty(bdi)))
+ return;
+
+ for (i = 0; i < NR_BDI_STAT_ITEMS; i++)
+ percpu_counter_init(&bdi->bdi_stat[i], 0);
+}
+EXPORT_SYMBOL(bdi_init);
> Probably the common
> case is writebackable backing-devs, so eliminating the test-n-branch might
> be a net microgain.
Time vs space. Now we don't even have storage for those BDIs..
Don't particularly care on this point though, I just thought it might be
worthwhile to save on the percpu data.
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