ppp: ppp_mp_explode() redesign

I found the PPP subsystem to not work properly when connecting channels
with different speeds to the same bundle.

Problem Description:

As the "ppp_mp_explode" function fragments the sk_buff buffer evenly
among the PPP channels that are connected to a certain PPP unit to
make up a bundle, if we are transmitting using an upper layer protocol
that requires an Ack before sending the next packet (like TCP/IP for
example), we will have a bandwidth bottleneck on the slowest channel
of the bundle.

Let's clarify by an example. Let's consider a scenario where we have
two PPP links making up a bundle: a slow link (10KB/sec) and a fast
link (1000KB/sec) working at the best (full bandwidth). On the top we
have a TCP/IP stack sending a 1000 Bytes sk_buff buffer down to the
PPP subsystem. The "ppp_mp_explode" function will divide the buffer in
two fragments of 500B each (we are neglecting all the headers, crc,
flags etc?.). Before the TCP/IP stack sends out the next buffer, it
will have to wait for the ACK response from the remote peer, so it
will have to wait for both fragments to have been sent over the two
PPP links, received by the remote peer and reconstructed. The
resulting behaviour is that, rather than having a bundle working
@1010KB/sec (the sum of the channels bandwidths), we'll have a bundle
working @20KB/sec (the double of the slowest channels bandwidth).


Problem Solution:

The problem has been solved by redesigning the "ppp_mp_explode"
function in such a way to make it split the sk_buff buffer according
to the speeds of the underlying PPP channels (the speeds of the serial
interfaces respectively attached to the PPP channels). Referring to
the above example, the redesigned "ppp_mp_explode" function will now
divide the 1000 Bytes buffer into two fragments whose sizes are set
according to the speeds of the channels where they are going to be
sent on (e.g .  10 Byets on 10KB/sec channel and 990 Bytes on
1000KB/sec channel).  The reworked function grants the same
performances of the original one in optimal working conditions (i.e. a
bundle made up of PPP links all working at the same speed), while
greatly improving performances on the bundles made up of channels
working at different speeds.
Signed-off-by: Gabriele Paoloni <gabriele.paoloni@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ppp_async.c

@@ -157,6 +157,7 @@ ppp_asynctty_open(struct tty_struct *tty)
 {
 	struct asyncppp *ap;
 	int err;
+	int speed;
 
 	if (tty->ops->write == NULL)
 		return -EOPNOTSUPP;
@@ -187,6 +188,8 @@ ppp_asynctty_open(struct tty_struct *tty)
 	ap->chan.private = ap;
 	ap->chan.ops = &async_ops;
 	ap->chan.mtu = PPP_MRU;
+	speed = tty_get_baud_rate(tty);
+	ap->chan.speed = speed;
 	err = ppp_register_channel(&ap->chan);
 	if (err)
 		goto out_free;

drivers/net/ppp_generic.c

@@ -167,6 +167,7 @@ struct channel {
 	u8		avail;		/* flag used in multilink stuff */
 	u8		had_frag;	/* >= 1 fragments have been sent */
 	u32		lastseq;	/* MP: last sequence # received */
+	int     speed;		/* speed of the corresponding ppp channel*/
 #endif /* CONFIG_PPP_MULTILINK */
 };
 
@@ -1307,35 +1308,49 @@ ppp_push(struct ppp *ppp)
  */
 static int ppp_mp_explode(struct ppp *ppp, struct sk_buff *skb)
 {
-	int len, fragsize;
+	int	len, totlen;
 	int	i, bits, hdrlen, mtu;
 	int	flen;
-	int navail, nfree;
+	int	navail,	nfree, nzero;
 	int	nbigger;
+	int	totspeed;
+	int	totfree;
 	unsigned char *p, *q;
 	struct list_head *list;
 	struct channel *pch;
 	struct sk_buff *frag;
 	struct ppp_channel *chan;
 
+	totspeed = 0; /*total bitrate of the bundle*/
 	nfree =	0;	/* # channels which	have no	packet already queued */
 	navail = 0;	/* total # of usable channels (not deregistered) */
+	nzero =	0; /* number of	channels with zero speed associated*/
+	totfree	= 0; /*total # of channels available and
+				  *having no queued packets before
+				  *starting the fragmentation*/
+
 	hdrlen = (ppp->flags & SC_MP_XSHORTSEQ)? MPHDRLEN_SSN: MPHDRLEN;
 	i =	0;
 	list_for_each_entry(pch, &ppp->channels, clist)	{
 		navail += pch->avail = (pch->chan != NULL);
+		pch->speed = pch->chan->speed;
 		if (pch->avail)	{
 			if (skb_queue_empty(&pch->file.xq) ||
 				!pch->had_frag)	{
+					if (pch->speed == 0)
+						nzero++;
+					else
+						totspeed += pch->speed;
+
 					pch->avail = 2;
 					++nfree;
+					++totfree;
 				}
 			if (!pch->had_frag && i	< ppp->nxchan)
 				ppp->nxchan	= i;
 		}
 		++i;
 	}
-
 	/*
 	 * Don't start sending this	packet unless at least half	of
 	 * the channels	are	free.  This	gives much better TCP
@@ -1352,18 +1367,7 @@ static int ppp_mp_explode(struct ppp *ppp, struct sk_buff *skb)
 		--len;
 	}
 
-	/*
-	 * Decide on fragment size.
-	 * We create a fragment for each free channel regardless of
-	 * how small they are (i.e. even 0 length) in order to minimize
-	 * the time that it will take to detect when a channel drops
-	 * a fragment.
-	 */
-	fragsize = len;
-	if (nfree > 1)
-		fragsize = DIV_ROUND_UP(fragsize, nfree);
-	/* nbigger channels get fragsize bytes, the rest get fragsize-1,
-	   except if nbigger==0, then they all get fragsize. */
+	totlen = len;
 	nbigger	= len %	nfree;
 
 	/* skip	to the channel after the one we	last used
@@ -1379,7 +1383,7 @@ static int ppp_mp_explode(struct ppp *ppp, struct sk_buff *skb)
 
 	/* create a	fragment for each channel */
 	bits = B;
-	while (nfree > 0 || len > 0) {
+	while (nfree > 0 &&	len	> 0) {
 		list = list->next;
 		if (list ==	&ppp->channels)	{
 			i =	0;
@@ -1398,7 +1402,6 @@ static int ppp_mp_explode(struct ppp *ppp, struct sk_buff *skb)
 			if (nfree >	0)
 				continue;
 		} else {
-			--nfree;
 			pch->avail = 1;
 		}
 
@@ -1406,28 +1409,69 @@ static int ppp_mp_explode(struct ppp *ppp, struct sk_buff *skb)
 		spin_lock_bh(&pch->downl);
 		if (pch->chan == NULL) {
 			/* can't use this channel, it's	being deregistered */
+			if (pch->speed == 0)
+				nzero--;
+			else
+				totspeed -=	pch->speed;
+
 			spin_unlock_bh(&pch->downl);
 			pch->avail = 0;
+			totlen = len;
+			totfree--;
+			nfree--;
 			if (--navail ==	0)
 				break;
 			continue;
 		}
 
 		/*
-		 * Create a fragment for this channel of
-		 * min(max(mtu+2-hdrlen, 4), fragsize, len) bytes.
-		 * If mtu+2-hdrlen < 4, that is a ridiculously small
-		 * MTU, so we use mtu = 2 + hdrlen.
-		 */
-		if (fragsize > len)
-			fragsize = len;
-		flen = fragsize;
+		*if the channel speed is not set divide
+		*the packet	evenly among the free channels;
+		*otherwise divide it according to the speed
+		*of the channel we are going to transmit on
+		*/
+		if (pch->speed == 0) {
+			flen = totlen/nfree	;
+			if (nbigger > 0) {
+				flen++;
+				nbigger--;
+			}
+		} else {
+			flen = (((totfree - nzero)*(totlen + hdrlen*totfree)) /
+				((totspeed*totfree)/pch->speed)) - hdrlen;
+			if (nbigger > 0) {
+				flen += ((totfree - nzero)*pch->speed)/totspeed;
+				nbigger -= ((totfree - nzero)*pch->speed)/
+							totspeed;
+			}
+		}
+		nfree--;
+
+		/*
+		 *check	if we are on the last channel or
+		 *we exceded the lenght	of the data	to
+		 *fragment
+		 */
+		if ((nfree == 0) || (flen > len))
+			flen = len;
+		/*
+		 *it is not worth to tx on slow channels:
+		 *in that case from the resulting flen according to the
+		 *above formula will be equal or less than zero.
+		 *Skip the channel in this case
+		 */
+		if (flen <=	0) {
+			pch->avail = 2;
+			spin_unlock_bh(&pch->downl);
+			continue;
+		}
+
 		mtu	= pch->chan->mtu + 2 - hdrlen;
 		if (mtu	< 4)
 			mtu	= 4;
 		if (flen > mtu)
 			flen = mtu;
-		if (flen == len && nfree == 0)
+		if (flen ==	len)
 			bits |=	E;
 		frag = alloc_skb(flen +	hdrlen + (flen == 0), GFP_ATOMIC);
 		if (!frag)
@@ -1447,18 +1491,6 @@ static int ppp_mp_explode(struct ppp *ppp, struct sk_buff *skb)
 			q[5] = ppp->nxseq;
 		}
 
-		/*
-		 * Copy the data in.
-		 * Unfortunately there is a bug in older versions of
-		 * the Linux PPP multilink reconstruction code where it
-		 * drops 0-length fragments.  Therefore we make sure the
-		 * fragment has at least one byte of data.  Any bytes
-		 * we add in this situation will end up as padding on the
-		 * end of the reconstructed packet.
-		 */
-		if (flen == 0)
-			*skb_put(frag, 1) = 0;
-		else
 		memcpy(q + hdrlen, p, flen);
 
 		/* try to send it down the channel */
@@ -1472,9 +1504,6 @@ static int ppp_mp_explode(struct ppp *ppp, struct sk_buff *skb)
 		++ppp->nxseq;
 		bits = 0;
 		spin_unlock_bh(&pch->downl);
-
-		if (--nbigger == 0 && fragsize > 0)
-			--fragsize;
 	}
 	ppp->nxchan	= i;
 

drivers/net/ppp_synctty.c

@@ -206,6 +206,7 @@ ppp_sync_open(struct tty_struct *tty)
 {
 	struct syncppp *ap;
 	int err;
+	int speed;
 
 	if (tty->ops->write == NULL)
 		return -EOPNOTSUPP;
@@ -234,6 +235,8 @@ ppp_sync_open(struct tty_struct *tty)
 	ap->chan.ops = &sync_ops;
 	ap->chan.mtu = PPP_MRU;
 	ap->chan.hdrlen = 2;	/* for A/C bytes */
+	speed = tty_get_baud_rate(tty);
+	ap->chan.speed = speed;
 	err = ppp_register_channel(&ap->chan);
 	if (err)
 		goto out_free;

include/linux/ppp_channel.h

@@ -40,8 +40,8 @@ struct ppp_channel {
 	int		mtu;		/* max transmit packet size */
 	int		hdrlen;		/* amount of headroom channel needs */
 	void		*ppp;		/* opaque to channel */
-	/* the following are not used at present */
 	int		speed;		/* transfer rate (bytes/second) */
+	/* the following is not used at present */
 	int		latency;	/* overhead time in milliseconds */
 };