netdev - Re: [RFC,PATCH] loopback: calls netif_receive_skb() instead of netif

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Date:	Sat, 01 Mar 2008 11:26:17 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	David Miller <davem@...emloft.net>
CC:	netdev@...r.kernel.org
Subject: Re: [RFC,PATCH] loopback: calls netif_receive_skb() instead of netif_rx()

David Miller a écrit :
> From: Eric Dumazet <dada1@...mosbay.com>
> Date: Thu, 21 Feb 2008 19:51:52 +0100
> 
>> Following patch directly calls netif_receive_skb() and avoids lot of 
>> atomic operations.
>> (atomic_inc(&dev->refcnt), set_and_set_bit(NAPI_STATE_SCHED, &n->state), ...
>>   atomic_dec(&dev->refcnt)...), cache line ping-pongs on device refcnt, 
>> but also softirq overhead.
>>
>> This gives a nice boost on tbench for example (5 % on my machine)
> 
> My only concern is stack usage.
> 
> Note that packet reception can elicit a response and go all the way
> back into this driver and all the way down into netif_receive_skb()
> again.  And so on and so forth.
> 
> If there is some bug in the stack (ACK'ing ACKs, stuff like that) we
> could get into a loop and overrun the kernel stack in no time at all.
> 
> So, if anything, this change could make inconvenient errors become
> catastrophic and hard to diagnose.

You are absolutly right. We should guard against recursion, using a new field 
in "pcpu_lstats" (cheap access in a hot cache line as we have to update stats 
anyway)

Thank you

[PATCH] loopback: calls netif_receive_skb() instead of netif_rx()

Loopback transmit function loopback_xmit() actually calls netif_rx() to queue 
a skb to the softnet queue, and arms a softirq so that this skb can be handled 
later.

This has a cost on SMP, because we need to hold a reference on the device, and 
free this reference when softirq dequeues packet.

Following patch directly calls netif_receive_skb() and avoids lot of atomic 
operations.

(atomic_inc(&dev->refcnt), set_and_set_bit(NAPI_STATE_SCHED, &n->state), ...
atomic_dec(&dev->refcnt)...), cache line ping-pongs on device refcnt, but also 
softirq overhead.

This gives a nice boost on tbench for example (5 % on my machine)

We want to limit recursion, in case network stack wants to re-enter 
loopback_xmit(). We use a depth field (per cpu), so that we avoid stack 
overflow, queueing the packet instead of trying to directly handle it.

Signed-off-by: Eric Dumazet <dada1@...mosbay.com>

View attachment "loopback.patch" of type "text/plain" (761 bytes)