Long-overdue MFC of r280930:

Long-overdue MFC of r280930:

  Fix overflow bugs in and remove obsolete limit from kernel RPC
  implementation.

  The kernel RPC code, which is responsible for the low-level scheduling
  of incoming NFS requests, contains a throttling mechanism that
  prevents too much kernel memory from being tied up by NFS requests
  that are being serviced.  When the throttle is engaged, the RPC layer
  stops servicing incoming NFS sockets, resulting ultimately in
  backpressure on the clients (if they're using TCP).  However, this is
  a very heavy-handed mechanism as it prevents all clients from making
  any requests, regardless of how heavy or light they are.  (Thus, when
  engaged, the throttle often prevents clients from even mounting the
  filesystem.)  The throttle mechanism applies specifically to requests
  that have been received by the RPC layer (from a TCP or UDP socket)
  and are queued waiting to be serviced by one of the nfsd threads; it
  does not limit the amount of backlog in the socket buffers.

  The original implementation limited the total bytes of queued requests
  to the minimum of a quarter of (nmbclusters * MCLBYTES) and 45 MiB.
  The former limit seems reasonable, since requests queued in the socket
  buffers and replies being constructed to the requests in progress will
  all require some amount of network memory, but the 45 MiB limit is
  plainly ridiculous for modern memory sizes: when running 256 service
  threads on a busy server, 45 MiB would result in just a single
  maximum-sized NFS3PROC_WRITE queued per thread before throttling.

  Removing this limit exposed integer-overflow bugs in the original
  computation, and related bugs in the routines that actually account
  for the amount of traffic enqueued for service threads.  The old
  implementation also attempted to reduce accounting overhead by
  batching updates until each queue is fully drained, but this is prone
  to livelock, resulting in repeated accumulate-throttle-drain cycles on
  a busy server.  Various data types are changed to long or unsigned
  long; explicit 64-bit types are not used due to the unavailability of
  64-bit atomics on many 32-bit platforms, but those platforms also
  cannot support nmbclusters large enough to cause overflow.

  This code (in a 10.1 kernel) is presently running on production NFS
  servers at CSAIL.

  Summary of this revision:
  * Removes 45 MiB limit on requests queued for nfsd service threads
  * Fixes integer-overflow and signedness bugs
  * Avoids unnecessary throttling by not deferring accounting for
    completed requests

Differential Revision:	https://reviews.freebsd.org/D2165
Reviewed by:	rmacklem, mav
Relnotes:	yes
Sponsored by:	MIT Computer Science & Artificial Intelligence Laboratory

git-svn-id: svn://svn.freebsd.org/base/stable/10@290203 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f