Message-ID: <4ACB6DE0.4010309@hartkopp.net>
Date:	Tue, 06 Oct 2009 18:18:40 +0200
From:	Oliver Hartkopp <socketcan@...tkopp.net>
To:	Neil Horman <nhorman@...driver.com>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] af_packet: add interframe drop cmsg (v6)

Neil Horman wrote:
> On Tue, Oct 06, 2009 at 03:42:26PM +0200, Eric Dumazet wrote:
>> 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 ;)
>>
> No I trust you.  The problem (as you just mentioned) is that you didn't parse my
> note.  We are saying the same thing.  If you want to implement this properly,
> you have to do exactly as you said, take a snapshot of the sk_drops value for
> the socket and store it in skb->mark, or some other unsued field, so that the
> drop information travels along the queue with the frames.  As I was reading
> Olivers note, it sounded to me as though he were proposing that we simply check
> sk_drops when calling sock_recvmsg, which doesn't implement the same sort of
> functionality.  I wanted to be sure that we were clear on what was trying to be
> done.

Puh - indeed much to read :-)

My intention was exactly your suggestion - as you also implemented the current
approach:

In socket_recvmsg the formerly stored (skb->mark) snapshot of sk_drops should
be used to create a cmsg, when the cmsg creation is enabled by a new setsockopt().

Did i get i right?

Regards,
Oliver

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