vhost: VPP stalls with vhost performing control plane actions

Symptom
-------
With NDR traffic blasting at VPP, bringing up a new VM with vhost
connection to VPP causes packet drops. I am able to recreate this
problem easily using a simple setup like this.

TREX-------------- switch ---- VPP
    |---------------|  |-------|

Cause
-----
The reason for the packet drops is due to vhost holding onto the worker
barrier lock for too long in vhost_user_socket_read(). There are quite a
few of system calls inside the routine. At the end of the routine, it
unconditionally calls vhost_user_update_iface_state() for all message
types. vhost_user_update_iface_state() also unconditionally calls
vhost_user_rx_thread_placement() and vhost_user_tx_thread_placement().
vhost_user_rx_thread_placement scraps out all existing cpu/queue mappings
for the interface and creates brand new cpu/queue mappings for the
interface. This process is very disruptive and very expensive. In my
opinion, this area of code needs a makeover.

Fixes
-----
* vhost_user_socket_read() is rewritten that it should not hold
  onto the worker barrier lock for system calls, or at least minimize the
  need for doing it.
* Remove the call to vhost_user_update_iface_state as a default route at
  the end of vhost_user_socket_read(). There is only a couple of message
  types which really need to call vhost_user_update_iface_state(). We put
  the call to those message types which need it.
* Remove vhost_user_rx_thread_placement() and
  vhost_user_tx_thread_placement from vhost_user_update_iface_state().
  There is no need to repetatively change the cpu/queue mappings.
* vhost_user_rx_thread_placement() is actually quite expensive. It should
  be called only once per queue for the interface. There is no need to
  scrap the existing cpu/queue mappings and create new cpu/queue mappings
  when the additional queues becomes active/enable.
* Change to create the cpu/queue mappings for the first RX when the
  interface is created. Dont remove the cpu/queue mapping when the
  interface is disconnected. Remove the cpu/queue mapping only when the
  interface is deleted.

The create vhost user interface CLI also has some very expensive system
calls if the command is entered with the optional keyword "server"

As a bonus, This patch makes the create vhost user interface binary-api and
CLI thread safe. Do the protection for the small amount of code which is
thread unsafe.

Change-Id: I4a19cbf7e9cc37ea01286169882e5603e6d7eb77
Signed-off-by: Steven Luong <sluong@cisco.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/vnet/devices/virtio/vhost_user_api.c b/src/vnet/devices/virtio/vhost_user_api.c
index b8d89a07cbc..acfb3e089f4 100644
--- a/src/vnet/devices/virtio/vhost_user_api.c
+++ b/src/vnet/devices/virtio/vhost_user_api.c
@@ -244,6 +244,9 @@ vhost_user_api_hookup (vlib_main_t * vm)
   foreach_vpe_api_msg;
 #undef _
 
+  /* Mark CREATE_VHOST_USER_IF as mp safe */
+  am->is_mp_safe[VL_API_CREATE_VHOST_USER_IF] = 1;
+
   /*
    * Set up the (msg_name, crc, message-id) table
    */