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Message-ID: <49C9D99A.2040900@cosmosbay.com>
Date: Wed, 25 Mar 2009 08:13:30 +0100
From: Eric Dumazet <dada1@...mosbay.com>
To: David Miller <davem@...emloft.net>
CC: netdev@...r.kernel.org
Subject: Re: [RFC] net: release dst entry in dev_queue_xmit()
David Miller a écrit :
> From: Eric Dumazet <dada1@...mosbay.com>
> Date: Fri, 20 Mar 2009 12:40:22 +0100
>
>> I believe we can release dst in dev_queue_xmit(), while cpu cache is
>> hot, since caller of dev_queue_xmit() had to hold a reference on dst
>> right before. This reduce work to be done by softirq handler, and
>> decrease cache misses.
>>
>
> This will break various packet schedulers and classifiers.
>
> Heck sch_sfq.c even uses skb->dst as part of it's flow hash
> function :-)
Well, as one of the hash perturbator, for other protocols than
IPV4 & IPV6...
default:
h = (unsigned long)skb->dst ^ skb->protocol;
h2 = (unsigned long)skb->sk;
}
return sfq_fold_hash(q, h, h2);
But teql indeed mandates dst in __teql_resolve()
Darn...
This dst freeing should be done very late then, in the NIC driver itself, just
before giving skb to hardware, or right before in dev_hard_start_xmit()
If done in dev_hard_start_xmit(), skb could be requeued (because of NETDEV_TX_BUSY).
Then if requeued, maybe at this time, dst being NULL is not a problem ?
Thanks a lot David
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