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

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Date:	Tue, 06 Oct 2009 18:24:12 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Oliver Hartkopp <socketcan@...tkopp.net>
CC:	Neil Horman <nhorman@...driver.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] af_packet: add interframe drop cmsg (v6)

Oliver Hartkopp a écrit :
> 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?
> 

Yes, I could spend a moment this evening to cook a patch if you want.


int on = 1;
setsockopt(fd, SOL_SOCKET, SO_COUNTDROPS, &on, sizeof(on));

diff --git a/include/asm-generic/socket.h b/include/asm-generic/socket.h
index 538991c..87996c3 100644
--- a/include/asm-generic/socket.h
+++ b/include/asm-generic/socket.h
@@ -63,4 +63,7 @@
 #define SO_PROTOCOL		38
 #define SO_DOMAIN		39
 
+#define SO_COUNTDROPS		40
+#define SCM_COUNTDROPS		SO_COUNTDROPS
+
 #endif /* __ASM_GENERIC_SOCKET_H */
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index df7b23a..4613ca4 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -390,8 +390,10 @@ struct sk_buff {
 	__u32			secmark;
 #endif
 
-	__u32			mark;
-
+	union {
+		__u32			mark;
+		__u32			dropcount;
+	};
 	__u16			vlan_tci;
 
 	sk_buff_data_t		transport_header;
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