linux-kernel - Re: NFS client growing system CPU

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Date:	Tue, 27 Sep 2011 13:04:15 -0400
From:	Trond Myklebust <Trond.Myklebust@...app.com>
To:	Simon Kirby <sim@...tway.ca>
Cc:	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: NFS client growing system CPU

On Tue, 2011-09-27 at 09:49 -0700, Simon Kirby wrote: 
> On Tue, Sep 27, 2011 at 07:42:53AM -0400, Trond Myklebust wrote:
> 
> > On Mon, 2011-09-26 at 17:39 -0700, Simon Kirby wrote: 
> > > Hello!
> > > 
> > > Following up on "System CPU increasing on idle 2.6.36", this issue is
> > > still happening even on 3.1-rc7. So, since it has been 9 months since I
> > > reported this, I figured I'd bisect this issue. The first bisection ended
> > > in an IPMI regression that looked like the problem, so I had to start
> > > again. Eventually, I got commit b80c3cb628f0ebc241b02e38dd028969fb8026a2
> > > which made it into 2.6.34-rc4.
> > > 
> > > With this commit, system CPU keeps rising as the log crunch box runs
> > > (reads log files via NFS and spews out HTML files into NFS-mounted report
> > > directories). When it finishes the daily run, the system time stays
> > > non-zero and continues to be higher and higher after each run, until the
> > > box never completes a run within a day due to all of the wasted cycles.
> > 
> > So reverting that commit fixes the problem on 3.1-rc7?
> > 
> > As far as I can see, doing so should be safe thanks to commit
> > 5547e8aac6f71505d621a612de2fca0dd988b439 (writeback: Update dirty flags
> > in two steps) which fixes the original problem at the VFS level.
> 
> Hmm, I went to git revert b80c3cb628f0ebc241b02e38dd028969fb8026a2, but
> for some reason git left the nfs_mark_request_dirty(req); line in
> nfs_writepage_setup(), even though the original commit had that. Is that
> OK or should I remove that as well?
> 
> Once that is sorted, I'll build it and let it run for a day and let you
> know. Thanks!

It shouldn't make any difference whether you leave it or remove it. The
resulting second call to __set_page_dirty_nobuffers() will always be a
no-op since the page will already be marked as dirty.

Cheers
  Trond
-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@...app.com
www.netapp.com

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