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

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Date:	Fri, 17 Aug 2012 21:26:27 +0400
From:	Michael Tokarev <mjt@....msk.ru>
To:	"J. Bruce Fields" <bfields@...ldses.org>
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 17.08.2012 21:18, J. Bruce Fields wrote:
> On Fri, Aug 17, 2012 at 09:12:38PM +0400, Michael Tokarev wrote:
[]
>> So we're calling svc_recv in a tight loop, eating
>> all available CPU.  (The above is with just 2 nfsd
>> threads).
>>
>> Something is definitely wrong here.  And it happens mure more
>> often after the mentioned commit (f03d78db65085).
> 
> Oh, neat.  Hm.  That commit doesn't really sound like the cause, then.
> Is that busy-looping reproduceable on kernels before that commit?

Note I bisected this issue to this commit.  I haven't seen it
happening before this commit, and reverting it from 3.0 or 3.2
kernel makes the problem to go away.

I guess it is looping there:


net/sunrpc/svc_xprt.c:svc_recv()
...
        len = 0;
...
        if (test_bit(XPT_LISTENER, &xprt->xpt_flags)) {
...
        } else if (xprt->xpt_ops->xpo_has_wspace(xprt)) {  <=== here -- has no wspace due to memory...
...  len = <something>
        }

        /* No data, incomplete (TCP) read, or accept() */
        if (len == 0 || len == -EAGAIN)
                goto out;
...
out:
        rqstp->rq_res.len = 0;
        svc_xprt_release(rqstp);
        return -EAGAIN;
}

I'm trying to verify this theory...

/mjt
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