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[RFC] macvlan: Handle broadcasts inline if we have only a few macvlans.

Message ID CAM_iQpWwX_tJVk-96v2Xjwrm+Nijk1AkRhe+aoNU6o_3bQh_BA@mail.gmail.com
State RFC, archived
Delegated to: David Miller
Headers show

Commit Message

Cong Wang May 27, 2016, 5:56 p.m. UTC 
On Thu, May 26, 2016 at 4:44 PM, Lennert Buytenhek
<buytenh@wantstofly.org> wrote:
> Commit 412ca1550cbecb2c ("macvlan: Move broadcasts into a work queue")
> moved processing of all macvlan multicasts into a work queue.  This
> causes a noticable performance regression when there is heavy multicast
> traffic on the underlying interface for multicast groups that the
> macvlan subinterfaces are not members of, in which case we end up
> cloning all those packets and then freeing them again from a work queue
> without really doing any useful work with them in between.

But we only queue up to 1000 packets in our backlog.

How about adding a quick check before cloning it?

                goto err;

Comments

Lennert Buytenhek May 27, 2016, 6:03 p.m. UTC | #1 
On Fri, May 27, 2016 at 10:56:44AM -0700, Cong Wang wrote:

> > Commit 412ca1550cbecb2c ("macvlan: Move broadcasts into a work queue")
> > moved processing of all macvlan multicasts into a work queue.  This
> > causes a noticable performance regression when there is heavy multicast
> > traffic on the underlying interface for multicast groups that the
> > macvlan subinterfaces are not members of, in which case we end up
> > cloning all those packets and then freeing them again from a work queue
> > without really doing any useful work with them in between.
> 
> But we only queue up to 1000 packets in our backlog.
> 
> How about adding a quick check before cloning it?
> 
> diff --git a/drivers/net/macvlan.c b/drivers/net/macvlan.c
> index cb01023..1c73d0f 100644
> --- a/drivers/net/macvlan.c
> +++ b/drivers/net/macvlan.c
> @@ -315,6 +315,9 @@ static void macvlan_broadcast_enqueue(struct
> macvlan_port *port,
>         struct sk_buff *nskb;
>         int err = -ENOMEM;
> 
> +       if (skb_queue_len(&port->bc_queue) >= MACVLAN_BC_QUEUE_LEN)
> +               return;
> +
>         nskb = skb_clone(skb, GFP_ATOMIC);
>         if (!nskb)
>                 goto err;

We're not hitting the bc_queue skb limit in our environment, as the
machine can keep up with the traffic -- it's just that taking an
extra clone of the skb and queueing and running the work queue item
to free it again is eating up a lot of cycles.

But doing the queue length check before the clone might not be a bad
idea?  (You'd probably want to atomic_long_inc(&skb->dev->rx_dropped)
before returning, though?)
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Patch

diff --git a/drivers/net/macvlan.c b/drivers/net/macvlan.c
index cb01023..1c73d0f 100644
--- a/drivers/net/macvlan.c
+++ b/drivers/net/macvlan.c
@@ -315,6 +315,9 @@  static void macvlan_broadcast_enqueue(struct
macvlan_port *port,
        struct sk_buff *nskb;
        int err = -ENOMEM;

+       if (skb_queue_len(&port->bc_queue) >= MACVLAN_BC_QUEUE_LEN)
+               return;
+
        nskb = skb_clone(skb, GFP_ATOMIC);
        if (!nskb)