netdev - Re: [PATCH 2/2] macvlan: Move broadcasts into a work queue

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Date:	Mon, 07 Apr 2014 07:07:42 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH 2/2] macvlan: Move broadcasts into a work queue

On Mon, 2014-04-07 at 15:55 +0800, Herbert Xu wrote:
> Currently broadcasts are handled in network RX context, where
> the packets are sent through netif_rx.  This means that the number
> of macvlans will be constrained by the capacity of netif_rx.
> 
> For example, setting up 4096 macvlans practically causes all
> broadcast packets to be dropped as the default netif_rx queue
> size simply can't handle 4096 skbs being stuffed into it all
> at once.
> 
> Fundamentally, we need to ensure that the amount of work handled
> in each netif_rx backlog run is constrained.  As broadcasts are
> anything but constrained, it either needs to be limited per run
> or moved to process context.
> 
> This patch picks the second option and moves all broadcast handling
> bar the trivial case of packets going to a single interface into
> a work queue.  Obviously there also needs to be a limit on how
> many broadcast packets we postpone in this way.  I've arbitrarily
> chosen tx_queue_len of the master device as the limit (act_mirred
> also happens to use this parameter in a similar way).
> 
> In order to ensure we don't exceed the backlog queue we will use
> netif_rx_ni instead of netif_rx for broadcast packets.
> 
> Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>
> ---
> 

Hi Herbert.

I suppose its a net-next material ?

Memory allocations (one incoming message -> ~4096 duplications) probably
should use GFP_KERNEL. This might need a change from rcu to simple mutex
for macvlan_broadcast() scan of all macvlans.

cond_resched() could help macvlan_process_broadcast() to not hog cpu.

Anyway, 4.000 incoming messages are duplicated into 16.000.000 messages,
it takes half a minute to process on a single cpu. You might need
multiple workqueue to split the load on all online cpus ;)

Thanks !

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