Re: GSS sequence number window

["J. Bruce Fields" <bfields@fieldses.org>]

On Tue, May 30, 2017 at 02:58:00PM -0400, Chuck Lever wrote:
> Hey Bruce!
> 
> While testing with sec=krb5 and sec=krb5i, I noticed a lot of
> spurious connection loss, especially when I wanted to run a
> CPU-intensive workload on my NFS server at the same time I
> was testing.
> 
> I added a pr_err() in gss_check_seq_num, and ran a fio job
> on a vers=3,sec=sys,proto=tcp mount (server is exporting a
> tmpfs). On the server, I rebuilt a kernel source tree cscope
> database at the same time.
> 
> May 29 17:53:13 klimt kernel: gss_check_seq_num: seq_num = 250098, sd_max = 250291, GSS_SEQ_WIN = 128
> May 29 17:53:33 klimt kernel: gss_check_seq_num: seq_num = 937816, sd_max = 938171, GSS_SEQ_WIN = 128
> May 29 17:53:33 klimt kernel: gss_check_seq_num: seq_num = 938544, sd_max = 938727, GSS_SEQ_WIN = 128
> May 29 17:53:33 klimt kernel: gss_check_seq_num: seq_num = 938543, sd_max = 938727, GSS_SEQ_WIN = 128
> May 29 17:53:34 klimt kernel: gss_check_seq_num: seq_num = 939344, sd_max = 939549, GSS_SEQ_WIN = 128
> May 29 17:53:35 klimt kernel: gss_check_seq_num: seq_num = 965007, sd_max = 965176, GSS_SEQ_WIN = 128
> May 29 17:54:01 klimt kernel: gss_check_seq_num: seq_num = 1799710, sd_max = 1799982, GSS_SEQ_WIN = 128
> May 29 17:54:02 klimt kernel: gss_check_seq_num: seq_num = 1831165, sd_max = 1831353, GSS_SEQ_WIN = 128
> May 29 17:54:04 klimt kernel: gss_check_seq_num: seq_num = 1883583, sd_max = 1883761, GSS_SEQ_WIN = 128
> May 29 17:54:07 klimt kernel: gss_check_seq_num: seq_num = 1959316, sd_max = 1959447, GSS_SEQ_WIN = 128
> 
> RFC 2203 suggests there's no risk to using a large window.
> My first thought was to make the sequence window larger
> (say 2048) but I've seen stragglers outside even that large
> a window.
> 
> Any thoughts about why there are these sequence number
> outliers?

No, alas.

The server's where it's easier to catch, but it'd be interesting to know
what's going on on the client side when this happens.

Might be interesting to know just what the operation (or compound ops)
are in the bad cases, in case there's some pattern.

Might also be interesting to check where exactly in the client code the
sequence number is assigned and where in the server code it's checked,
and think about where they might get queued up or reordered in between.

Is it possible there are retries involved?

Does increasing the number of server threads help?

I don't think it'd be a problem to increase the sequence window.

--b.