OPEN_XOR_DELEGATION performance problems

[Jeff Layton <jlayton@kernel.org>]

We attempted to implement the "delstid" draft for v6.13, but have had
to drop the patches for it. After merge, we got a couple of reports of
a performance issue due to the OPEN_XOR_DELEGATION patch:

    https://lore.kernel.org/linux-nfs/202409161645.d44bced5-oliver.sang@intel.com/

Once we enable OPEN_XOR_DELEGATION support, the fsmark "App Overhead"
statistic spikes significantly. The kernel patch for this is very
simple, and doesn't seem likely to cause a performance issue on its
own. My theory is that this test is one that causes the client to
return the delegation, and since it doesn't have an open stateid, it
has to reestablish one during the test run, and that causes the app
overhead stat to spike.

Trond, Tom, Mike -- I know that the HS Anvil has support for
OPEN_XOR_DELEGATION. If you run the fsmark test against it with that
support both enabled and disabled (either on the client or server
side), do you see a similar spike in "App Overhead"?

If so, then I suspect we need to consider limiting the use of that flag
in some cases. I have no idea what heuristic we'd use to decide this
though.
-- 
Jeff Layton <jlayton@kernel.org>