netdev - Re: vxlan/veth performance issues on net.git + latest kernels

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Date:	Tue, 3 Dec 2013 16:55:33 -0800
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Or Gerlitz <or.gerlitz@...il.com>,
	David Miller <davem@...emloft.net>,
	Joseph Gasparakis <joseph.gasparakis@...el.com>,
	Jerry Chu <hkchu@...gle.com>,
	Or Gerlitz <ogerlitz@...lanox.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Pravin B Shelar <pshelar@...ira.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: vxlan/veth performance issues on net.git + latest kernels

On Tue, Dec 3, 2013 at 4:36 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2013-12-03 at 16:26 -0800, Alexei Starovoitov wrote:
>
>>
>> Could you use some other way to mark skb ?
>
> I could ;)
>
>> In tracing we might want to examine skb more carefully and not being
>> able to see the device
>> will limit the usability of this tracepoint.
>
> Unfortunately, using skb->dev as a pointer to device would be buggy or
> expensive (you would need to take a reference on device in order not
> letting it disappear, as we escape RCU protection)

well, yes, you might have an skb around when device is already freed
when skb_dst_noref.
but I'm not suggesting anything expensive. Tracing definitely should
not add overhead by doing rcu_lock() or dev_hold(). Instead it can go
through skb, skb->dev, skb->dev->xxx via probe_kernel_read(). If dev
is gone, it's still safe.

> Anyway, this magic is pretty easy to change, I am open to suggestions.

you're the expert :) use skb->mark field, since it's unused during
freeing path... ?
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