[ovs-dev] [PATCH v2 2/3] dpif-netdev: Avoid port's reconfiguration on pmd-cpu-mask changes.

Ilya Maximets i.maximets at samsung.com
Fri Jul 7 06:11:29 UTC 2017


On 07.07.2017 08:08, Darrell Ball wrote:
> 
> 
> On 5/30/17, 7:12 AM, "Ilya Maximets" <i.maximets at samsung.com> wrote:
> 
>     Reconfiguration of HW NICs may lead to packet drops.
>     In current model all physical ports will be reconfigured each
>     time number of PMD threads changed. Since we not stopping
>     threads on pmd-cpu-mask changes, this patch will help to further
>     decrease port's downtime by setting the maximum possible number
>     of wanted tx queues to avoid unnecessary reconfigurations.
>     
>     Signed-off-by: Ilya Maximets <i.maximets at samsung.com>
>     ---
>      lib/dpif-netdev.c | 26 +++++++++++++++++++++-----
>      1 file changed, 21 insertions(+), 5 deletions(-)
>     
>     diff --git a/lib/dpif-netdev.c b/lib/dpif-netdev.c
>     index 596d133..79db770 100644
>     --- a/lib/dpif-netdev.c
>     +++ b/lib/dpif-netdev.c
>     @@ -3453,7 +3453,7 @@ reconfigure_datapath(struct dp_netdev *dp)
>      {
>          struct dp_netdev_pmd_thread *pmd;
>          struct dp_netdev_port *port;
>     -    int wanted_txqs;
>     +    int needed_txqs, wanted_txqs;
>      
>          dp->last_reconfigure_seq = seq_read(dp->reconfigure_seq);
>      
>     @@ -3461,7 +3461,15 @@ reconfigure_datapath(struct dp_netdev *dp)
>           * on the system and the user configuration. */
>          reconfigure_pmd_threads(dp);
>      
>     -    wanted_txqs = cmap_count(&dp->poll_threads);
>     +    /* We need 1 Tx queue for each thread to avoid locking, but we will try
>     +     * to allocate the maximum possible value to minimize the number of port
>     +     * reconfigurations. */
>     +    needed_txqs = cmap_count(&dp->poll_threads);
>     +    /* (n_cores + 1) is the maximum that we might need to have.
>     +     * Additional queue is for non-PMD threads. */
>     +    wanted_txqs = ovs_numa_get_n_cores();
>     +    ovs_assert(wanted_txqs != OVS_CORE_UNSPEC);
>     +    wanted_txqs++;
> 
> I don’t think PMD mask changes are common, so this patch is trying to optimize a 
> rare disruptive event that can/will be scheduled by the administrator.
> 
> Based on the actual number of queues supported and the number of cores present,
> this optimization may or may not work. It is unpredictable whether there will be benefit
> in a particular case from the user POV.
> If I were the administrator, I would try to error on the conservative side anyways if I could
> not predict the result.
> 
> Did I miss something ?

In NFV environment if you want to add one more VM to your hosts you will have to
choose between:

	* not creating a new PMD thread
		-> performance degradation of networking for other working VMs

	* adding of the new PMD thread
		-> desruption of the networking for the whole host for the time
		   of OVS reconfiguration.

This patch removes the cost of the second option.

I don't understand what you mean saying 'unpredictable benefit'. The benefit is clear
and this optimization will work, except for HW NICs with very low number of HW queues.
But situation will never be worse than without this patch anyway.

About PMD mask changes:
Even one change in a week can be disruptive in high-loaded NFV environment.
Also, this changes can be performed automatically by monitoring tools while
VMs migrations or rebalancing the load between PMD threads to free the CPU
cores for other applications/VMs.

> 
>          /* The number of pmd threads might have changed, or a port can be new:
>           * adjust the txqs. */
>     @@ -3474,9 +3482,17 @@ reconfigure_datapath(struct dp_netdev *dp)
>      
>          /* Check for all the ports that need reconfiguration.  We cache this in
>           * 'port->reconfigure', because netdev_is_reconf_required() can change at
>     -     * any time. */
>     +     * any time.
>     +     * Also mark for reconfiguration all ports which will likely change their
>     +     * 'dynamic_txqs' parameter. It's required to stop using them before
>     +     * changing this setting and it's simpler to mark ports here and allow
>     +     * 'pmd_remove_stale_ports' to remove them from threads. There will be
>     +     * no actual reconfiguration in 'port_reconfigure' because it's
>     +     * unnecessary.  */
>          HMAP_FOR_EACH (port, node, &dp->ports) {
>     -        if (netdev_is_reconf_required(port->netdev)) {
>     +        if (netdev_is_reconf_required(port->netdev)
>     +            || (port->dynamic_txqs
>     +                !=  netdev_n_txq(port->netdev) < needed_txqs)) {
>                  port->need_reconfigure = true;
>              }
>          }
>     @@ -3510,7 +3526,7 @@ reconfigure_datapath(struct dp_netdev *dp)
>                  seq_change(dp->port_seq);
>                  port_destroy(port);
>              } else {
>     -            port->dynamic_txqs = netdev_n_txq(port->netdev) < wanted_txqs;
>     +            port->dynamic_txqs = netdev_n_txq(port->netdev) < needed_txqs;
>              }
>          }
>      
>     -- 
>     2.7.4
>     
>     
> 


More information about the dev mailing list