netdev - Re: [PATCH] af_packet: add interframe drop cmsg (v6)

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Date:	Tue, 06 Oct 2009 15:42:26 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Neil Horman <nhorman@...driver.com>
CC:	Oliver Hartkopp <socketcan@...tkopp.net>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] af_packet: add interframe drop cmsg (v6)

Neil Horman a écrit :
>
> Actually, no, I don't think this is sufficient.  Looking at how the
> implementation would work, we would query the sk_drop value in the recvmsg path,
> but we wouldn't record it during frame enqueue, which would skew the data I need
> to record.  Consider a queue of holding frames with sequence 1 2 3 4 101 102
> 110.  Assume that missing sequence numbers were dropped during enqueue to the
> sk_recieve_queue for lack of space.  Using the proposed above implementation,
> the frames the gap gets reported with is determined by the scheduling of the
> user space app, rather than the actual order in which they were lost.  I.e if
> frames 5-100 were received & dropped immediately after the user process called
> recvmsg for the first time, we would get the cmsg notification when we called
> recvmsg and read frame 2, but if we lost those frames after the process called
> recvmsg twice, we'd recognize the loss when we read frame 3.  With my current
> implementation we recognize the gap on the next enqueued frame after we
> encountered the loss, keeping everything in order, which is important.  We would
> need to do that with whatever socket level option we'd consider implementing.
> Not saying its not doable, but we need to take it into account, and its not a
> straightforward as simply reading sk_drops on recvmsg.  We'd have to store the
> sk_drop value in skb->mask universally or some such, and then teach sock_recvmsg
> to check that field.
> 
>

Sorry cannot parse this, its too long to be true :)

If you count every af_packet drops in sk_drops, and every time you _enqueue_
a frame in sk->receive_queue, you copy sk->sk_drops in skb->mark, you implement
the thing. All this is protected by a lock.

sk->receive_queue is a FIFO, so there is no loss of information.

You only need to increment sk->sk_drops when necessary, if not already done in af_packet code.

If you dont trust me I can provide patch ;)

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