mirror of
git://projects.qi-hardware.com/openwrt-xburst.git
synced 2024-08-20 02:23:22 +03:00
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@31336 3c298f89-4303-0410-b956-a3cf2f4a3e73
189 lines
7.5 KiB
Diff
189 lines
7.5 KiB
Diff
From 9d02daf754238adac48fa075ee79e7edd3d79ed3 Mon Sep 17 00:00:00 2001
|
|
From: David Woodhouse <dwmw2@infradead.org>
|
|
Date: Sun, 8 Apr 2012 09:55:43 +0000
|
|
Subject: [PATCH] pppoatm: Fix excessive queue bloat
|
|
|
|
We discovered that PPPoATM has an excessively deep transmit queue. A
|
|
queue the size of the default socket send buffer (wmem_default) is
|
|
maintained between the PPP generic core and the ATM device.
|
|
|
|
Fix it to queue a maximum of *two* packets. The one the ATM device is
|
|
currently working on, and one more for the ATM driver to process
|
|
immediately in its TX done interrupt handler. The PPP core is designed
|
|
to feed packets to the channel with minimal latency, so that really
|
|
ought to be enough to keep the ATM device busy.
|
|
|
|
While we're at it, fix the fact that we were triggering the wakeup
|
|
tasklet on *every* pppoatm_pop() call. The comment saying "this is
|
|
inefficient, but doing it right is too hard" turns out to be overly
|
|
pessimistic... I think :)
|
|
|
|
On machines like the Traverse Geos, with a slow Geode CPU and two
|
|
high-speed ADSL2+ interfaces, there were reports of extremely high CPU
|
|
usage which could partly be attributed to the extra wakeups.
|
|
|
|
(The wakeup handling could actually be made a whole lot easier if we
|
|
stop checking sk->sk_sndbuf altogether. Given that we now only queue
|
|
*two* packets ever, one wonders what the point is. As it is, you could
|
|
already deadlock the thing by setting the sk_sndbuf to a value lower
|
|
than the MTU of the device, and it'd just block for ever.)
|
|
|
|
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
|
|
Signed-off-by: David S. Miller <davem@davemloft.net>
|
|
---
|
|
net/atm/pppoatm.c | 95 +++++++++++++++++++++++++++++++++++++++++++++++-----
|
|
1 files changed, 85 insertions(+), 10 deletions(-)
|
|
|
|
--- a/net/atm/pppoatm.c
|
|
+++ b/net/atm/pppoatm.c
|
|
@@ -62,12 +62,25 @@ struct pppoatm_vcc {
|
|
void (*old_pop)(struct atm_vcc *, struct sk_buff *);
|
|
/* keep old push/pop for detaching */
|
|
enum pppoatm_encaps encaps;
|
|
+ atomic_t inflight;
|
|
+ unsigned long blocked;
|
|
int flags; /* SC_COMP_PROT - compress protocol */
|
|
struct ppp_channel chan; /* interface to generic ppp layer */
|
|
struct tasklet_struct wakeup_tasklet;
|
|
};
|
|
|
|
/*
|
|
+ * We want to allow two packets in the queue. The one that's currently in
|
|
+ * flight, and *one* queued up ready for the ATM device to send immediately
|
|
+ * from its TX done IRQ. We want to be able to use atomic_inc_not_zero(), so
|
|
+ * inflight == -2 represents an empty queue, -1 one packet, and zero means
|
|
+ * there are two packets in the queue.
|
|
+ */
|
|
+#define NONE_INFLIGHT -2
|
|
+
|
|
+#define BLOCKED 0
|
|
+
|
|
+/*
|
|
* Header used for LLC Encapsulated PPP (4 bytes) followed by the LCP protocol
|
|
* ID (0xC021) used in autodetection
|
|
*/
|
|
@@ -102,16 +115,30 @@ static void pppoatm_wakeup_sender(unsign
|
|
static void pppoatm_pop(struct atm_vcc *atmvcc, struct sk_buff *skb)
|
|
{
|
|
struct pppoatm_vcc *pvcc = atmvcc_to_pvcc(atmvcc);
|
|
+
|
|
pvcc->old_pop(atmvcc, skb);
|
|
+ atomic_dec(&pvcc->inflight);
|
|
+
|
|
/*
|
|
- * We don't really always want to do this since it's
|
|
- * really inefficient - it would be much better if we could
|
|
- * test if we had actually throttled the generic layer.
|
|
- * Unfortunately then there would be a nasty SMP race where
|
|
- * we could clear that flag just as we refuse another packet.
|
|
- * For now we do the safe thing.
|
|
+ * We always used to run the wakeup tasklet unconditionally here, for
|
|
+ * fear of race conditions where we clear the BLOCKED flag just as we
|
|
+ * refuse another packet in pppoatm_send(). This was quite inefficient.
|
|
+ *
|
|
+ * In fact it's OK. The PPP core will only ever call pppoatm_send()
|
|
+ * while holding the channel->downl lock. And ppp_output_wakeup() as
|
|
+ * called by the tasklet will *also* grab that lock. So even if another
|
|
+ * CPU is in pppoatm_send() right now, the tasklet isn't going to race
|
|
+ * with it. The wakeup *will* happen after the other CPU is safely out
|
|
+ * of pppoatm_send() again.
|
|
+ *
|
|
+ * So if the CPU in pppoatm_send() has already set the BLOCKED bit and
|
|
+ * it about to return, that's fine. We trigger a wakeup which will
|
|
+ * happen later. And if the CPU in pppoatm_send() *hasn't* set the
|
|
+ * BLOCKED bit yet, that's fine too because of the double check in
|
|
+ * pppoatm_may_send() which is commented there.
|
|
*/
|
|
- tasklet_schedule(&pvcc->wakeup_tasklet);
|
|
+ if (test_and_clear_bit(BLOCKED, &pvcc->blocked))
|
|
+ tasklet_schedule(&pvcc->wakeup_tasklet);
|
|
}
|
|
|
|
/*
|
|
@@ -184,6 +211,51 @@ error:
|
|
ppp_input_error(&pvcc->chan, 0);
|
|
}
|
|
|
|
+static inline int pppoatm_may_send(struct pppoatm_vcc *pvcc, int size)
|
|
+{
|
|
+ /*
|
|
+ * It's not clear that we need to bother with using atm_may_send()
|
|
+ * to check we don't exceed sk->sk_sndbuf. If userspace sets a
|
|
+ * value of sk_sndbuf which is lower than the MTU, we're going to
|
|
+ * block for ever. But the code always did that before we introduced
|
|
+ * the packet count limit, so...
|
|
+ */
|
|
+ if (atm_may_send(pvcc->atmvcc, size) &&
|
|
+ atomic_inc_not_zero_hint(&pvcc->inflight, NONE_INFLIGHT))
|
|
+ return 1;
|
|
+
|
|
+ /*
|
|
+ * We use test_and_set_bit() rather than set_bit() here because
|
|
+ * we need to ensure there's a memory barrier after it. The bit
|
|
+ * *must* be set before we do the atomic_inc() on pvcc->inflight.
|
|
+ * There's no smp_mb__after_set_bit(), so it's this or abuse
|
|
+ * smp_mb__after_clear_bit().
|
|
+ */
|
|
+ test_and_set_bit(BLOCKED, &pvcc->blocked);
|
|
+
|
|
+ /*
|
|
+ * We may have raced with pppoatm_pop(). If it ran for the
|
|
+ * last packet in the queue, *just* before we set the BLOCKED
|
|
+ * bit, then it might never run again and the channel could
|
|
+ * remain permanently blocked. Cope with that race by checking
|
|
+ * *again*. If it did run in that window, we'll have space on
|
|
+ * the queue now and can return success. It's harmless to leave
|
|
+ * the BLOCKED flag set, since it's only used as a trigger to
|
|
+ * run the wakeup tasklet. Another wakeup will never hurt.
|
|
+ * If pppoatm_pop() is running but hasn't got as far as making
|
|
+ * space on the queue yet, then it hasn't checked the BLOCKED
|
|
+ * flag yet either, so we're safe in that case too. It'll issue
|
|
+ * an "immediate" wakeup... where "immediate" actually involves
|
|
+ * taking the PPP channel's ->downl lock, which is held by the
|
|
+ * code path that calls pppoatm_send(), and is thus going to
|
|
+ * wait for us to finish.
|
|
+ */
|
|
+ if (atm_may_send(pvcc->atmvcc, size) &&
|
|
+ atomic_inc_not_zero(&pvcc->inflight))
|
|
+ return 1;
|
|
+
|
|
+ return 0;
|
|
+}
|
|
/*
|
|
* Called by the ppp_generic.c to send a packet - returns true if packet
|
|
* was accepted. If we return false, then it's our job to call
|
|
@@ -207,7 +279,7 @@ static int pppoatm_send(struct ppp_chann
|
|
struct sk_buff *n;
|
|
n = skb_realloc_headroom(skb, LLC_LEN);
|
|
if (n != NULL &&
|
|
- !atm_may_send(pvcc->atmvcc, n->truesize)) {
|
|
+ !pppoatm_may_send(pvcc, n->truesize)) {
|
|
kfree_skb(n);
|
|
goto nospace;
|
|
}
|
|
@@ -215,12 +287,12 @@ static int pppoatm_send(struct ppp_chann
|
|
skb = n;
|
|
if (skb == NULL)
|
|
return DROP_PACKET;
|
|
- } else if (!atm_may_send(pvcc->atmvcc, skb->truesize))
|
|
+ } else if (!pppoatm_may_send(pvcc, skb->truesize))
|
|
goto nospace;
|
|
memcpy(skb_push(skb, LLC_LEN), pppllc, LLC_LEN);
|
|
break;
|
|
case e_vc:
|
|
- if (!atm_may_send(pvcc->atmvcc, skb->truesize))
|
|
+ if (!pppoatm_may_send(pvcc, skb->truesize))
|
|
goto nospace;
|
|
break;
|
|
case e_autodetect:
|
|
@@ -285,6 +357,9 @@ static int pppoatm_assign_vcc(struct atm
|
|
if (pvcc == NULL)
|
|
return -ENOMEM;
|
|
pvcc->atmvcc = atmvcc;
|
|
+
|
|
+ /* Maximum is zero, so that we can use atomic_inc_not_zero() */
|
|
+ atomic_set(&pvcc->inflight, NONE_INFLIGHT);
|
|
pvcc->old_push = atmvcc->push;
|
|
pvcc->old_pop = atmvcc->pop;
|
|
pvcc->encaps = (enum pppoatm_encaps) be.encaps;
|