Messages in this thread | | | Subject | Re: [PATCH 08/10] mm: count writeback pages per BDI | From | Peter Zijlstra <> | Date | Sat, 21 Apr 2007 13:07:16 +0200 |
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On Sat, 2007-04-21 at 02:55 -0700, Andrew Morton wrote: > On Fri, 20 Apr 2007 17:52:02 +0200 Peter Zijlstra <a.p.zijlstra@chello.nl> wrote: > > > Count per BDI writeback pages. > > > > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> > > --- > > 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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