Users of the Geos platform are reporting high CPU utilization. This seems to be
rooted in a problem with the TX queue restart in PPP.
Signed-off-by: Philip Prindeville <phil...@redfish-solutions.com>
Index: target/linux/generic/patches-3.2/120-ppp_txqueue_restart.patch
===================================================================
--- target/linux/generic/patches-3.2/120-ppp_txqueue_restart.patch
(revision 0)
+++ target/linux/generic/patches-3.2/120-ppp_txqueue_restart.patch
(revision 0)
@@ -0,0 +1,82 @@
+For every transmitted packet, ppp_start_xmit() will stop the netdev
+queue and then, if appropriate, restart it. This causes the TX softirq
+to run, entirely gratuitously.
+
+This is "only" a waste of CPU time in the normal case, but it's actively
+harmful when the PPP device is a TEQL slave — the wakeup will cause the
+offending device to receive the next TX packet from the TEQL queue, when
+it *should* have gone to the next slave in the list. We end up seeing
+large bursts of packets on just *one* slave device, rather than using
+the full available bandwidth over all slaves.
+
+This patch fixes the problem by *not* unconditionally stopping the queue
+in ppp_start_xmit(). It adds a return value from ppp_xmit_process()
+which indicates whether the queue should be stopped or not.
+
+It *doesn't* remove the call to netif_wake_queue() from
+ppp_xmit_process(), because other code paths (especially from
+ppp_output_wakeup()) need it there and it's messy to push it out to the
+other callers to do it based on the return value. So we leave it in
+place — it's a no-op in the case where the queue wasn't stopped, so it's
+harmless in the TX path.
+
+Signed-off-by: David Woodhouse <david.woodho...@intel.com>
+
+--- drivers/net/ppp/ppp_generic.c~ 2012-01-26 00:39:32.000000000 +0000
++++ drivers/net/ppp/ppp_generic.c 2012-03-26 10:32:31.286744147 +0100
+@@ -235,7 +235,7 @@ struct ppp_net {
+ /* Prototypes. */
+ static int ppp_unattached_ioctl(struct net *net, struct ppp_file *pf,
+ struct file *file, unsigned int cmd, unsigned long arg);
+-static void ppp_xmit_process(struct ppp *ppp);
++static int ppp_xmit_process(struct ppp *ppp);
+ static void ppp_send_frame(struct ppp *ppp, struct sk_buff *skb);
+ static void ppp_push(struct ppp *ppp);
+ static void ppp_channel_push(struct channel *pch);
+@@ -968,9 +968,9 @@ ppp_start_xmit(struct sk_buff *skb, stru
+ proto = npindex_to_proto[npi];
+ put_unaligned_be16(proto, pp);
+
+- netif_stop_queue(dev);
+ skb_queue_tail(&ppp->file.xq, skb);
+- ppp_xmit_process(ppp);
++ if (!ppp_xmit_process(ppp))
++ netif_stop_queue(dev);
+ return NETDEV_TX_OK;
+
+ outf:
+@@ -1048,10 +1048,11 @@ static void ppp_setup(struct net_device
+ * Called to do any work queued up on the transmit side
+ * that can now be done.
+ */
+-static void
++static int
+ ppp_xmit_process(struct ppp *ppp)
+ {
+ struct sk_buff *skb;
++ int ret = 0;
+
+ ppp_xmit_lock(ppp);
+ if (!ppp->closing) {
+@@ -1061,10 +1062,13 @@ ppp_xmit_process(struct ppp *ppp)
+ ppp_send_frame(ppp, skb);
+ /* If there's no work left to do, tell the core net
+ code that we can accept some more. */
+- if (!ppp->xmit_pending && !skb_peek(&ppp->file.xq))
++ if (!ppp->xmit_pending && !skb_peek(&ppp->file.xq)) {
+ netif_wake_queue(ppp->dev);
++ ret = 1;
++ }
+ }
+ ppp_xmit_unlock(ppp);
++ return ret;
+ }
+
+ static inline struct sk_buff *
+
+--
+David Woodhouse Open Source Technology Centre
+david.woodho...@intel.com Intel Corporation
+
+
+
_______________________________________________
openwrt-devel mailing list
openwrt-devel@lists.openwrt.org
https://lists.openwrt.org/mailman/listinfo/openwrt-devel
