netdev - Re: can we reuse an skb

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Date:	Mon, 22 Jun 2009 19:26:32 +0530
From:	Peter Chacko <peterchacko35@...il.com>
To:	Philby John <pjohn@...mvista.com>
Cc:	Nicholas Van Orton <turanammo@...il.com>, jon_zhou@...lent.com,
	radhamohan_ch@...oo.com, netdev@...r.kernel.org
Subject: Re: can we reuse an skb

Philby,

I thank you much for your time helping me out, answering me.

My intention here is to customize packet buffer allocation for
special case, when the linux box in question is just packet processor.
 That i don't want to allocate memory from a common pool for common
purpose, like slab cached , re-usable objects like skb. I want to have
finer control of the memory access time(by allocating objects from L1
cache, and  keeping it around as fixed no of  packet buffers, like in
a typical routers.

I just want to know whether i can re-use any body's work /a patch
available in this goal, before  i embark on making custom code.

As you said, i will down-load the most updated code and correct my
self, if there are enough optimizations available already.

Thanks
Peter chacko,


On Mon, Jun 22, 2009 at 7:04 PM, Philby John<pjohn@...mvista.com> wrote:
> On Fri, 2009-06-19 at 15:41 +0530, Nicholas Van Orton wrote:
>> Does this mean that when skb buffer has been allocated using
>> dev_alloc_skb(), filled with received data and passed to the upper
>> layers
>> the kernel would automatically do the task of releasing this buffer
>> without the user calling dev_kfree_skb()?
>
> Yes, I think that is the case. Except when the user calls an ioctl that
> closes your ethernet device, by say using $ifconfig eth0 down, in which
> case you must free the ring skb buffer's allocated using
> dev_kfree_skb().
>
>>  I once got
>> KERNEL: assertion (!atomic_read(&skb->users)) failed at net/core/dev.c
>> errors when trying
>> to free them using dev_kfree_skb()
>>
>> Could this be cause I did not wait until netif_rx_completed() was called?
>
> You are using an old version of the kernel, can't see such code in
> 2.6.30. From what I know, this usually happens if skb->users is not
> equal to one, which means the buffer is in use by some user. Like I
> said, you needn't call dev_kfree_skb() explicitly, it will be freed
> after use by the upper network layers.
>
> netif_receive_skb() ->deliver_skb()-> pt_prev->func() ->
> ip_rcv() -> ip_rcv_finish()
>
> ip_rcv_finish() would finally free it as per the specified protocol.
> This I think is the flow, but I guess there would be experts here who
> would correct me if I am wrong.
>
> -Philby
>
>
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