linux-kernel - Re: 3.0+ NFS issues (bisected)

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Date:	Sat, 18 Aug 2012 07:13:54 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Michael Tokarev <mjt@....msk.ru>
Cc:	"Myklebust, Trond" <Trond.Myklebust@...app.com>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	Linux-kernel <linux-kernel@...r.kernel.org>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: 3.0+ NFS issues (bisected)

On Sat, Aug 18, 2012 at 10:49:31AM +0400, Michael Tokarev wrote:
> On 18.08.2012 02:32, J. Bruce Fields wrote:
> > On Fri, Aug 17, 2012 at 04:08:07PM -0400, J. Bruce Fields wrote:
> >> Wait a minute, that assumption's a problem because that calculation
> >> depends in part on xpt_reserved, which is changed here....
> >>
> >> In particular, svc_xprt_release() calls svc_reserve(rqstp, 0), which
> >> subtracts rqstp->rq_reserved and then calls svc_xprt_enqueue, now with a
> >> lower xpt_reserved value.  That could well explain this.
> > 
> > So, maybe something like this?
> 
> Well.  What can I say?  With the change below applied (to 3.2 kernel
> at least), I don't see any stalls or high CPU usage on the server
> anymore.  It survived several multi-gigabyte transfers, for several
> hours, without any problem.  So it is a good step forward ;)
> 
> But the whole thing seems to be quite a bit fragile.  I tried to follow
> the logic in there, and the thing is quite a bit, well, "twisted", and
> somewhat difficult to follow.  So I don't know if this is the right
> fix or not.  At least it works! :)

Suggestions welcomed.

> And I really wonder why no one else reported this problem before.
> Is me the only one in this world who uses linux nfsd? :)

This, for example:

	http://marc.info/?l=linux-nfs&m=134131915612287&w=2

may well describe the same problem....  It just needed some debugging
persistence, thanks!

--b.
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