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

[Ilya Maximets]

On 07.07.2017 21:09, Darrell Ball wrote:
> 
> 
> On 7/6/17, 11:11 PM, "Ilya Maximets" <i.maximets at samsung.com> wrote:
> 
>     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.
> 
> In general, adding a PMD thread per VM may not always (or even usually) make sense.

Not per VM. Lets assume that all existing threads are already overloaded.

> There are use cases for sure, but using a PMD core per VM is often viewed as dpdk using too much
> cpu resources and limiting the adoption of dpdk.
> 
> Furthermore, for dpdk gateways it is irrelevant.

Disagree with that statement.

>     
>     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.
> 
> HW nic interfaces carry aggregated traffic for multiple VMs etc, so these cases are most important.

And that is the point why this patch implemented. This patch tries to
minimize the packet drops on the "shared" between VMs HW NICs. It
allows to add new threads without breaking networking for others not
reconfiguring device that handles traffic for other VMs at this moment.

> It is the ratio of the number of cores to hw queues that matter. If queues are less than
> cores, then the benefit may not occur.

That's right. But are you saying that we should not apply optimization if
it covers only 50% of cases? (There is bigger percentage, actually, because
modern NICs supports pretty much tx queues)

> This patch makes assumptions about the relative number of cores vs the number of queues.
> These two parameters should be treated as independent – this part really bothers me.

It doesn't make any assumptions, it just optimizes one of 2 possible situations.
If the number of queues will be less, the behaviour will be exactly same as in
current master.

> I think we could solve this in a coordinated generic fashion with a user management command
> that would also give feedback to the user as to the possible success and maybe provide options.
> I wonder if we will eventually go in that direction, in the future, anyways ?

I don't think so. We don't need to have user management command because the
only sane values for the number of queues right now is the number of threads
or the maximum number of available queues in hardware. We're using the
first one only because HW is not able to say how many queues it able to
successfully configure.
This patch allows us to use the second value in all cases where possible.

> 
>     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
>     >     
>     >     
>     > 
>     
>