linux-kernel - Re: [LKP] [lkp] [xfs] 68a9f5e700: aim7.jobs-per-min -13.6% regression

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Message-ID: <20160811155721.GA23015@lst.de>
Date:	Thu, 11 Aug 2016 17:57:21 +0200
From:	Christoph Hellwig <hch@....de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Huang, Ying" <ying.huang@...el.com>,
	Dave Chinner <david@...morbit.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Bob Peterson <rpeterso@...hat.com>,
	Wu Fengguang <fengguang.wu@...el.com>, LKP <lkp@...org>,
	Christoph Hellwig <hch@....de>
Subject: Re: [LKP] [lkp] [xfs] 68a9f5e700: aim7.jobs-per-min -13.6%
	regression

On Wed, Aug 10, 2016 at 06:00:24PM -0700, Linus Torvalds wrote:
> The biggest difference is that we have "mark_page_accessed()" show up
> after, and not before. There was also a lot of LRU noise in the
> non-profile data. I wonder if that is the reason here: the old model
> of using generic_perform_write/block_page_mkwrite didn't mark the
> pages accessed, and now with iomap_file_buffered_write() they get
> marked as active and that screws up the LRU list, and makes us not
> flush out the dirty pages well (because they are seen as active and
> not good for writeback), and then you get bad memory use.

And that's actually a "bug" in the new code - mostly because I failed
to pick up changes to the core code happening after we 'forked' it,
in this case commit 2457ae ("mm: non-atomically mark page accessed during page
cache allocation where possible").

The one liner below (not tested yet) to simply remove it should fix that
up.  I also noticed we have a spurious pagefault_disable/enable, I
need to dig into the history of that first, though.

diff --git a/fs/iomap.c b/fs/iomap.c
index 48141b8..f39c318 100644
--- a/fs/iomap.c
+++ b/fs/iomap.c
@@ -199,7 +199,6 @@ again:
 		pagefault_enable();
 
 		flush_dcache_page(page);
-		mark_page_accessed(page);
 
 		status = iomap_write_end(inode, pos, bytes, copied, page);
 		if (unlikely(status < 0))