linux-kernel - Re: [PATCH 08/10] mm: count writeback pages per BDI

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Message-Id: <1177153636.2934.43.camel@lappy>
Date:	Sat, 21 Apr 2007 13:07:16 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	miklos@...redi.hu, neilb@...e.de, dgc@....com,
	tomoki.sekiyama.qu@...achi.com, nikita@...sterfs.com,
	trond.myklebust@....uio.no, yingchao.zhou@...il.com
Subject: Re: [PATCH 08/10] mm: count writeback pages per BDI

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@...llo.nl> wrote:
> 
> > Count per BDI writeback pages.
> > 
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.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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