Re: NFS server regression in kernel 5.13 (tested w/ 5.13.9)

[Daire Byrne <daire@dneg.com>]

This sounds a bit like my bugzilla ticket:
https://bugzilla.kernel.org/show_bug.cgi?id=213887

But in my case the client rarely recovers from the timeout so the hang
becomes permanent. My setup is a bit more complicated though (NFS
re-exporting).

Daire

On Tue, 10 Aug 2021 at 02:29, NeilBrown <neilb@suse.de> wrote:
>
> On Tue, 10 Aug 2021, Mike Javorski wrote:
> > Managed to get the rpcdebug info for some freezes today. I had a loop
> > running to call the rpcbind command that Neil provided and then sleep
> > for 2 seconds. Each triggering event was the opening of a new video
> > file (I was shuffling through files with mpv). I haven't managed to
> > get a tcp capture yet trying to get one that isn't too massive, but
> > the timing is tricky, and this doesn't happen every time.
> >
>
> Thanks for the logs.
>
> They show consistent "a:call_status" and "q:xprt_pending".  They also
> show a timeout in the range 10000-20000.  This number is in 'jiffies',
> which is normally millisecs, so 10 to 20 seconds.
>
> That means the RPC layer (thinks it has) sent a request (made a call)
> and is waiting for a reply to arrive over the network transport (i.e.
> the TCP connection).  However that would typically involve a much longer
> timeout.  For NFSv3 and 4.0, 60 seconds is the norm.  For NFSv4.1 I
> *think* there is no timeout as NFS never resends requests unless the TCP
> connection is broken.  So I don't know what to make of that.
>
> Given the fact that it recovers in 5-10 seconds, it seems very likely
> that it actually has sent the request (presumably the timeout doesn't
> expire).  Mostly likely the server is being delayed in replying.  A
> tcpdump trace would confirm this.
>
> I don't think you've said anything about what NFS server you are using.
> It is Linux, or something else?  If Linux, what kernel do you run there?
>
> One thing that might cause a delay when accessing a file is if some
> other client has a 'delegation' and takes a while to return it.
> e.g.
>   client2 creates a file for writing
>   server gives it a 'write delegation'
>   client2 queues up lots of write request
>   client1 asks to read the file.
>   server says to client2 "please return the delegation"
>   client2 starts flushing its data, but either takes too long or
>    has some other hiccup. Eventually it is returned, or the server
>    revokes it
>   server tells clients one it can read now.
>
> This is leading to me asking: do you have other NFS clients that might
> be accessing the same file, or might have accessed it recently?
>
> NeilBrown
>
>
> > -----------------------------
> >
> > [31229.965368] -pid- flgs status -client- --rqstp- -timeout ---ops--
> > [31229.965370] 44224 c801      0 8c478d70 2473aa3b    15395  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31229.965411] 44225 c801      0 8c478d70 a7d3b9b8    15395  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31231.966982] -pid- flgs status -client- --rqstp- -timeout ---ops--
> > [31231.966986] 44224 c801      0 8c478d70 2473aa3b    14795  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31231.967027] 44225 c801      0 8c478d70 a7d3b9b8    14795  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31233.968741] -pid- flgs status -client- --rqstp- -timeout ---ops--
> > [31233.968745] 44224 c801      0 8c478d70 2473aa3b    14194  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31233.968787] 44225 c801      0 8c478d70 a7d3b9b8    14194  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> >
> > -----------------------------
> >
> > [31292.534529] -pid- flgs status -client- --rqstp- -timeout ---ops--
> > [31292.534530] 45744 c801      0 8c478d70 2473aa3b    15340  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31292.534554] 45745 c801      0 8c478d70 50cbef7b    15342  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31294.536203] -pid- flgs status -client- --rqstp- -timeout ---ops--
> > [31294.536206] 45744 c801      0 8c478d70 2473aa3b    14740  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31294.536232] 45745 c801      0 8c478d70 50cbef7b    14742  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31296.537826] -pid- flgs status -client- --rqstp- -timeout ---ops--
> > [31296.537830] 45744 c801      0 8c478d70 2473aa3b    14139  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31296.537871] 45745 c801      0 8c478d70 50cbef7b    14141  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31298.539474] -pid- flgs status -client- --rqstp- -timeout ---ops--
> > [31298.539478] 45744 c801      0 8c478d70 2473aa3b    13539  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31298.539522] 45745 c801      0 8c478d70 50cbef7b    13541  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31300.541040] -pid- flgs status -client- --rqstp- -timeout ---ops--
> > [31300.541044] 46048 c801      0 8c478d70 50cbef7b    17965  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31300.541108] 46049 c801      0 8c478d70 2473aa3b    17965  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31302.542632] -pid- flgs status -client- --rqstp- -timeout ---ops--
> > [31302.542635] 46922 c801      0 8c478d70 50cbef7b    18000  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> >
> > ---------------------------
> >
> > [31375.046020] -pid- flgs status -client- --rqstp- -timeout ---ops--
> > [31375.046022] 47336 c801      0 8c478d70 cebaa9a6    15964  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31375.046048] 47337 c801      0 8c478d70 f7145d4d    15964  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31377.047361] -pid- flgs status -client- --rqstp- -timeout ---ops--
> > [31377.047365] 47336 c801      0 8c478d70 cebaa9a6    15364  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31377.047406] 47337 c801      0 8c478d70 f7145d4d    15364  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31379.048801] -pid- flgs status -client- --rqstp- -timeout ---ops--
> > [31379.048805] 47336 c801      0 8c478d70 cebaa9a6    14764  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31379.048845] 47337 c801      0 8c478d70 f7145d4d    14764  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31381.050376] -pid- flgs status -client- --rqstp- -timeout ---ops--
> > [31381.050380] 47336 c801      0 8c478d70 cebaa9a6    14163  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31381.050421] 47337 c801      0 8c478d70 f7145d4d    14163  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31383.051746] -pid- flgs status -client- --rqstp- -timeout ---ops--
> > [31383.051749] 47336 c801      0 8c478d70 cebaa9a6    13563  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31383.051775] 47337 c801      0 8c478d70 f7145d4d    13563  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31385.053182] -pid- flgs status -client- --rqstp- -timeout ---ops--
> > [31385.053185] 47342 c801      0 8c478d70 f7145d4d    17980  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31387.054422] -pid- flgs status -client- --rqstp- -timeout ---ops--
> > [31387.054426] 47818 c801      0 8c478d70 f7145d4d    17761  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> > [31387.054467] 47819 c801      0 8c478d70 cebaa9a6    17762  8531859
> > nfsv4 READ a:call_status [sunrpc] q:xprt_pending
> >
> >
> > - mike
> >
> >
> > On Sun, Aug 8, 2021 at 5:28 PM Mike Javorski <mike.javorski@gmail.com> wrote:
> > >
> > > Appreciate the insights Neil. I will make an attempt to
> > > trigger/capture the fault if I can.
> > >
> > > - mike
> > >
> > > On Sun, Aug 8, 2021 at 5:01 PM NeilBrown <neilb@suse.de> wrote:
> > > >
> > > > On Mon, 09 Aug 2021, Mike Javorski wrote:
> > > > > I have been experiencing nfs file access hangs with multiple release
> > > > > versions of the 5.13.x linux kernel. In each case, all file transfers
> > > > > freeze for 5-10 seconds and then resume. This seems worse when reading
> > > > > through many files sequentially.
> > > >
> > > > A particularly useful debugging tool for NFS freezes is to run
> > > >
> > > >   rpcdebug -m rpc -c all
> > > >
> > > > while the system appears frozen.  As you only have a 5-10 second window
> > > > this might be tricky.
> > > > Setting or clearing debug flags in the rpc module (whether they are
> > > > already set or not) has a side effect if listing all RPC "tasks" which a
> > > > waiting for a reply.  Seeing that task list can often be useful.
> > > >
> > > > The task list appears in "dmesg" output.  If there are not tasks
> > > > waiting, nothing will be written which might lead you to think it didn't
> > > > work.
> > > >
> > > > As Chuck hinted, tcpdump is invaluable for this sort of problem.
> > > >   tcpdump -s 0 -w /tmp/somefile.pcap port 2049
> > > >
> > > > will capture NFS traffic.  If this can start before a hang, and finish
> > > > after, it may contain useful information.  Doing that in a way that
> > > > doesn't create an enormous file might be a challenge.  It would help if
> > > > you found a way trigger the problem.  Take note of the circumstances
> > > > when it seems to happen the most.  If you can only produce a large file,
> > > > we can probably still work with it.
> > > >   tshark -r /tmp/somefile.pcap
> > > > will report the capture one line per packet.  You can look for the
> > > > appropriate timestamp, note the frame numbers, and use "editcap"
> > > > to extract a suitable range of packets.
> > > >
> > > > NeilBrown
> > > >
> > > >
> > > > >
> > > > > My server:
> > > > > - Archlinux w/ a distribution provided kernel package
> > > > > - filesystems exported with "rw,sync,no_subtree_check,insecure" options
> > > > >
> > > > > Client:
> > > > > - Archlinux w/ latest distribution provided kernel (5.13.9-arch1-1 at writing)
> > > > > - nfs mounted via /net autofs with "soft,nodev,nosuid" options
> > > > > (ver=4.2 is indicated in mount)
> > > > >
> > > > > I have tried the 5.13.x kernel several times since the first arch
> > > > > release (most recently with 5.13.9-arch1-1), all with similar results.
> > > > > Each time, I am forced to downgrade the linux package to a 5.12.x
> > > > > kernel (5.12.15-arch1 as of writing) to clear up the transfer issues
> > > > > and stabilize performance. No other changes are made between tests. I
> > > > > have confirmed the freezing behavior using both ext4 and btrfs
> > > > > filesystems exported from this server.
> > > > >
> > > > > At this point I would appreciate some guidance in what to provide in
> > > > > order to diagnose and resolve this issue. I don't have a lot of kernel
> > > > > debugging experience, so instruction would be helpful.
> > > > >
> > > > > - mike
> > > > >
> > > > >
> >
> >