Re: [PATCH] net: reduce number of reference taken on sk_refcnt

[David Miller <davem@davemloft.net>]

From: Eric Dumazet <dada1@cosmosbay.com>
Date: Sat, 09 May 2009 14:13:59 +0200

> David Miller a écrit :
>> From: Eric Dumazet <dada1@cosmosbay.com>
>> Date: Fri, 08 May 2009 17:12:39 +0200
>> 
>>> For example, we can avoid the dst_release() cache miss if this
>>> is done in start_xmit(), and not later in TX completion while freeing skb.
>>> I tried various patches in the past but unfortunatly it seems
>>> only safe way to do this is in the driver xmit itself, not in core
>>> network stack. This would need many patches, one for each driver.
>> 
>> There might be a way around having to hit every driver.
>> 
>> The case we can't muck with is when the route will be used.
>> Devices which create this kind of situation can be marked with
>> a flag bit in struct netdevice.  If that flag bit isn't set,
>> you can drop the DST in dev_hard_start_xmit().
> 
> Yes, this is a possibility, I'll think about it, thank you.
> I'll have to recall which devices would need this flag (loopback for sure)..

Another idea is to invert the flag, and set it in places such
as alloc_etherdev*().

This might speed up getting coverage for the most obvious cases.