linux-kernel - Re: [PATCH v9 20/25] ext4: Convert from readpages to readahead

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Message-ID: <20200320184041.GG851@sol.localdomain>
Date:   Fri, 20 Mar 2020 11:40:41 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-xfs@...r.kernel.org,
        William Kucharski <william.kucharski@...cle.com>,
        linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, cluster-devel@...hat.com,
        linux-mm@...ck.org, ocfs2-devel@....oracle.com,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-erofs@...ts.ozlabs.org, linux-btrfs@...r.kernel.org
Subject: Re: [PATCH v9 20/25] ext4: Convert from readpages to readahead

On Fri, Mar 20, 2020 at 10:48:48AM -0700, Matthew Wilcox wrote:
> On Fri, Mar 20, 2020 at 10:37:34AM -0700, Eric Biggers wrote:
> > On Fri, Mar 20, 2020 at 07:22:26AM -0700, Matthew Wilcox wrote:
> > > From: "Matthew Wilcox (Oracle)" <willy@...radead.org>
> > > 
> > > Use the new readahead operation in ext4
> > > 
> > > Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> > > Reviewed-by: William Kucharski <william.kucharski@...cle.com>
> > > ---
> > >  fs/ext4/ext4.h     |  3 +--
> > >  fs/ext4/inode.c    | 21 +++++++++------------
> > >  fs/ext4/readpage.c | 22 ++++++++--------------
> > >  3 files changed, 18 insertions(+), 28 deletions(-)
> > > 
> > 
> > Reviewed-by: Eric Biggers <ebiggers@...gle.com>
> > 
> > > +		if (rac) {
> > > +			page = readahead_page(rac);
> > >  			prefetchw(&page->flags);
> > > -			list_del(&page->lru);
> > > -			if (add_to_page_cache_lru(page, mapping, page->index,
> > > -				  readahead_gfp_mask(mapping)))
> > > -				goto next_page;
> > >  		}
> > 
> > Maybe the prefetchw(&page->flags) should be included in readahead_page()?
> > Most of the callers do it.
> 
> I did notice that a lot of callers do that.  I wonder whether it (still)
> helps or whether it's just cargo-cult programming.  It can't possibly
> have helped before because we did list_del(&page->lru) as the very next
> instruction after prefetchw(), and they're in the same cacheline.  It'd
> be interesting to take it out and see what happens to performance.

Yeah, it does look like the list_del() made the prefetchw() useless, so it
should just be removed.  The prefetchw() dates all the way back to
mpage_readpages() being added in 2002, but even then the list_del() was
immediately afterwards, and 'flags' and 'lru' were in the same cache line in
'struct page' even then (assuming at least a 32-byte cache line size), so...

- Eric