netdev - Re: [PATCH v2] netdev:bfin_mac: reclaim and free tx skb as soon as possible after transfer

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Date:	Fri, 4 Jun 2010 12:44:02 +0800
From:	Sonic Zhang <sonic.adi@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	uclinux-dist-devel <uclinux-dist-devel@...ckfin.uclinux.org>
Subject: Re: [PATCH v2] netdev:bfin_mac: reclaim and free tx skb as soon as 
	possible after transfer

On Fri, Jun 4, 2010 at 12:05 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le vendredi 04 juin 2010 à 11:29 +0800, Sonic Zhang a écrit :
>> David,
>>
>> Any comments?
>>
>> Thanks
>>
>> Sonic
>>
>> On Thu, Jun 3, 2010 at 11:48 AM, sonic zhang <sonic.adi@...il.com> wrote:
>> > >From 40560ae9e8db42e2d2259b791ace160534c9a0f2 Mon Sep 17 00:00:00 2001
>> > From: Sonic Zhang <sonic.zhang@...log.com>
>> > Date: Thu, 3 Jun 2010 11:44:33 +0800
>> > Subject: [PATCH v2] netdev:bfin_mac: reclaim and free tx skb as soon as possible after transfer
>> >
>> > SKBs hold onto resources that can't be held indefinitely, such as TCP
>> > socket references and netfilter conntrack state.  So if a packet is left
>> > in TX ring for a long time, there might be a TCP socket that cannot be
>> > closed and freed up.
>> >
>> > Current blackfin EMAC driver always reclaim and free used tx skbs in future
>> > transfers. The problem is that future transfer may not come as soon as
>> > possible. This patch start a timer after transfer to reclaim and free skb.
>> > There is nearly no performance drop with this patch.
>> >
>> > TX interrupt is not enabled for 2 reasons:
>> >
>> > 1) If Blackfin EMAC TX transfer control is turned on, endless TX
>> > interrupts are triggered no matter if TX DMA is enabled. Since DMA walks
>> > down the ring automatically, TX transfer control can't be turned off in the
>> > middle. The only way is to disable TX interrupt completely.
>> >
>> > 2) skb can not be freed from interrupt context. A work queue or tasklet
>> > has to be created, which introduce more overhead than timer only solution.
>> >
>
> Could you elaborate on this second point ?
>
> skb can be freed from interrupt context using appropriate API :
>
> 1) If from NAPI context, no special care is needed and use
> dev_kfree_skb().
>
> 2) If from hard irq context, use dev_kfree_skb_irq() :
>

Yes, you are right. dev_kfree_skb_irq() queues used skb to the
complete queue. But, it is actually freed in the other soft irq
NET_TX_SOFTIRQ.


> With recent changes, skb is probably already orphaned and can be freed
> immediately.
>
> In the unlikely case it is not yet orphaned, skb is queued in
> softnet_data.completion_queue.
>
>
>
>
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