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Date:	Thu, 21 Aug 2014 14:57:20 -0700
From:	Benjamin Poirier <bpoirier@...e.de>
To:	Prashant Sreedharan <prashant@...adcom.com>,
	Michael Chan <mchan@...adcom.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH v2 3/3] tg3: Fix tx_pending checks for tg3_tso_bug

In tg3_set_ringparam(), the tx_pending test to cover the cases where
tg3_tso_bug() is entered has two problems
1) the check is only done for certain hardware whereas the workaround
is now used more broadly. IOW, the check may not be performed when it
is needed.
2) the check is too optimistic.

For example, with a 5761 (SHORT_DMA_BUG), tg3_set_ringparam() skips over the
"tx_pending <= (MAX_SKB_FRAGS * 3)" check because TSO_BUG is false. Even if it
did do the check, with a full sized skb, frag_cnt_est = 135 but the check is
for <= MAX_SKB_FRAGS * 3 (= 17 * 3 = 51). So the check is insufficient. This
leads to the following situation: by setting, ex. tx_pending = 100, there can
be an skb that triggers tg3_tso_bug() and that is large enough to cause
tg3_tso_bug() to stop the queue even when it is empty. We then end up with a
netdev watchdog transmit timeout.

Given that 1) some of the conditions tested for in tg3_tx_frag_set() apply
regardless of the chipset flags and that 2) it is difficult to estimate ahead
of time the max possible number of frames that a large skb may be split into
by gso, we instead take the approach of adjusting dev->gso_max_segs according
to the requested tx_pending size.

This puts us in the exceptional situation that a single skb that triggers
tg3_tso_bug() may require the entire tx ring. Usually the tx queue is woken up
when at least a quarter of it is available (TG3_TX_WAKEUP_THRESH) but that
would be insufficient now. To avoid useless wakeups, the tx queue wake up
threshold is made dynamic. Likewise, usually the tx queue is stopped as soon
as an skb with max frags may overrun it. Since the skbs submitted from
tg3_tso_bug() use a controlled number of descriptors, the tx queue stop
threshold may be lowered.

Signed-off-by: Benjamin Poirier <bpoirier@...e.de>
---
Changes v1->v2
* in tg3_set_ringparam(), reduce gso_max_segs further to budget 3 descriptors
  per gso seg instead of only 1 as in v1
* in tg3_tso_bug(), check that this estimation (3 desc/seg) holds, otherwise
  linearize some skbs as needed
* in tg3_start_xmit(), make the queue stop threshold a parameter, for the
  reason explained in the commit description

I was concerned that this last change, because of the extra call in the
default xmit path, may impact performance so I performed an rr latency test
but I did not measure a significant impact. That test was with default mtu and
ring size.

# perf stat -r10 -ad netperf -H 192.168.9.30 -l60 -T 0,0 -t omni -- -d rr

* without patches
	rr values: 7039.63 6865.03 6939.21 6919.31 6931.88 6932.74 6925.1 6953.33 6868.43 6935.65
	sample size: 10
	mean: 6931.031
	standard deviation: 48.10918
	quantiles: 6865.03 6920.757 6932.31 6938.32 7039.63
	6930±50

 Performance counter stats for 'netperf -H 192.168.9.30 -l60 -T 0,0 -t omni -- -d rr' (10 runs):

     480643.024723 task-clock                #    8.001 CPUs utilized            ( +-  0.00% ) [100.00%]
           855,136 context-switches          #    0.002 M/sec                    ( +-  0.23% ) [100.00%]
               521 CPU-migrations            #    0.000 M/sec                    ( +-  6.49% ) [100.00%]
               104 page-faults               #    0.000 M/sec                    ( +-  2.73% )
   298,416,906,437 cycles                    #    0.621 GHz                      ( +-  4.08% ) [15.01%]
   812,072,320,370 stalled-cycles-frontend   #  272.13% frontend cycles idle     ( +-  1.89% ) [25.01%]
   685,633,562,247 stalled-cycles-backend    #  229.76% backend  cycles idle     ( +-  2.50% ) [35.00%]
   117,665,891,888 instructions              #    0.39  insns per cycle
                                             #    6.90  stalled cycles per insn  ( +-  2.22% ) [45.00%]
    26,158,399,505 branches                  #   54.424 M/sec                    ( +-  2.10% ) [50.00%]
       205,688,614 branch-misses             #    0.79% of all branches          ( +-  0.78% ) [50.00%]
    27,882,474,171 L1-dcache-loads           #   58.011 M/sec                    ( +-  1.98% ) [50.00%]
       369,911,372 L1-dcache-load-misses     #    1.33% of all L1-dcache hits    ( +-  0.62% ) [50.00%]
        76,240,847 LLC-loads                 #    0.159 M/sec                    ( +-  1.04% ) [40.00%]
             3,220 LLC-load-misses           #    0.00% of all LL-cache hits     ( +- 19.49% ) [ 5.00%]

      60.074059340 seconds time elapsed                                          ( +-  0.00% )

* with patches
	rr values: 6732.65 6920.1 6909.46 7032.41 6864.43 6897.6 6815.19 6967.83 6849.23 6929.52
	sample size: 10
	mean: 6891.842
	standard deviation: 82.91901
	quantiles: 6732.65 6853.03 6903.53 6927.165 7032.41
	6890±80

 Performance counter stats for 'netperf -H 192.168.9.30 -l60 -T 0,0 -t omni -- -d rr' (10 runs):

     480675.949728 task-clock                #    8.001 CPUs utilized            ( +-  0.01% ) [100.00%]
           850,461 context-switches          #    0.002 M/sec                    ( +-  0.37% ) [100.00%]
               564 CPU-migrations            #    0.000 M/sec                    ( +-  5.67% ) [100.00%]
               417 page-faults               #    0.000 M/sec                    ( +- 76.04% )
   287,019,442,295 cycles                    #    0.597 GHz                      ( +-  7.16% ) [15.01%]
   828,198,830,689 stalled-cycles-frontend   #  288.55% frontend cycles idle     ( +-  3.01% ) [25.01%]
   718,230,307,166 stalled-cycles-backend    #  250.24% backend  cycles idle     ( +-  3.53% ) [35.00%]
   117,976,598,188 instructions              #    0.41  insns per cycle
                                             #    7.02  stalled cycles per insn  ( +-  4.06% ) [45.00%]
    26,715,853,108 branches                  #   55.580 M/sec                    ( +-  3.77% ) [50.00%]
       198,787,673 branch-misses             #    0.74% of all branches          ( +-  0.86% ) [50.00%]
    28,416,922,166 L1-dcache-loads           #   59.119 M/sec                    ( +-  3.54% ) [50.00%]
       367,613,007 L1-dcache-load-misses     #    1.29% of all L1-dcache hits    ( +-  0.47% ) [50.00%]
        75,260,575 LLC-loads                 #    0.157 M/sec                    ( +-  2.24% ) [40.00%]
             5,777 LLC-load-misses           #    0.01% of all LL-cache hits     ( +- 36.03% ) [ 5.00%]

      60.077898757 seconds time elapsed                                          ( +-  0.01% )

I reproduced this bug using the same approach explained in patch 1.
The bug reproduces with tx_pending <= 135

---
 drivers/net/ethernet/broadcom/tg3.c | 67 +++++++++++++++++++++++++++++--------
 drivers/net/ethernet/broadcom/tg3.h |  1 +
 2 files changed, 54 insertions(+), 14 deletions(-)

diff --git a/drivers/net/ethernet/broadcom/tg3.c b/drivers/net/ethernet/broadcom/tg3.c
index 0cecd6d..c29f2e3 100644
--- a/drivers/net/ethernet/broadcom/tg3.c
+++ b/drivers/net/ethernet/broadcom/tg3.c
@@ -204,6 +204,10 @@ static inline void _tg3_flag_clear(enum TG3_FLAGS flag, unsigned long *bits)
 /* minimum number of free TX descriptors required to wake up TX process */
 #define TG3_TX_WAKEUP_THRESH(tnapi)	max_t(u32, (tnapi)->tx_pending / 4, \
 					      MAX_SKB_FRAGS + 1)
+/* estimate a certain number of descriptors per gso segment */
+#define TG3_TX_DESC_PER_SEG(seg_nb)	((seg_nb) * 3)
+#define TG3_TX_SEG_PER_DESC(desc_nb)	((desc_nb) / 3)
+
 #define TG3_TX_BD_DMA_MAX_2K		2048
 #define TG3_TX_BD_DMA_MAX_4K		4096
 
@@ -6609,10 +6613,10 @@ static void tg3_tx(struct tg3_napi *tnapi)
 	smp_mb();
 
 	if (unlikely(netif_tx_queue_stopped(txq) &&
-		     (tg3_tx_avail(tnapi) > TG3_TX_WAKEUP_THRESH(tnapi)))) {
+		     (tg3_tx_avail(tnapi) > tnapi->wakeup_thresh))) {
 		__netif_tx_lock(txq, smp_processor_id());
 		if (netif_tx_queue_stopped(txq) &&
-		    (tg3_tx_avail(tnapi) > TG3_TX_WAKEUP_THRESH(tnapi)))
+		    (tg3_tx_avail(tnapi) > tnapi->wakeup_thresh))
 			netif_tx_wake_queue(txq);
 		__netif_tx_unlock(txq);
 	}
@@ -7830,6 +7834,8 @@ static int tigon3_dma_hwbug_workaround(struct tg3_napi *tnapi,
 }
 
 static netdev_tx_t tg3_start_xmit(struct sk_buff *, struct net_device *);
+static netdev_tx_t __tg3_start_xmit(struct sk_buff *, struct net_device *,
+				    u32);
 
 /* Use GSO to workaround all TSO packets that meet HW bug conditions
  * indicated in tg3_tx_frag_set()
@@ -7838,11 +7844,13 @@ static int tg3_tso_bug(struct tg3 *tp, struct tg3_napi *tnapi,
 		       struct netdev_queue *txq, struct sk_buff *skb)
 {
 	struct sk_buff *segs, *nskb;
-	u32 frag_cnt_est = skb_shinfo(skb)->gso_segs * 3;
+	unsigned int segs_remaining = skb_shinfo(skb)->gso_segs;
+	u32 desc_cnt_est = TG3_TX_DESC_PER_SEG(segs_remaining);
 
-	/* Estimate the number of fragments in the worst case */
-	if (unlikely(tg3_tx_avail(tnapi) <= frag_cnt_est)) {
+	if (unlikely(tg3_tx_avail(tnapi) <= desc_cnt_est)) {
 		netif_tx_stop_queue(txq);
+		tnapi->wakeup_thresh = desc_cnt_est;
+		BUG_ON(tnapi->wakeup_thresh >= tnapi->tx_pending);
 
 		/* netif_tx_stop_queue() must be done before checking
 		 * checking tx index in tg3_tx_avail() below, because in
@@ -7850,7 +7858,7 @@ static int tg3_tso_bug(struct tg3 *tp, struct tg3_napi *tnapi,
 		 * netif_tx_queue_stopped().
 		 */
 		smp_mb();
-		if (tg3_tx_avail(tnapi) <= frag_cnt_est)
+		if (tg3_tx_avail(tnapi) <= tnapi->wakeup_thresh)
 			return NETDEV_TX_BUSY;
 
 		netif_tx_wake_queue(txq);
@@ -7858,14 +7866,33 @@ static int tg3_tso_bug(struct tg3 *tp, struct tg3_napi *tnapi,
 
 	segs = skb_gso_segment(skb, tp->dev->features &
 				    ~(NETIF_F_TSO | NETIF_F_TSO6));
-	if (IS_ERR(segs) || !segs)
+	if (IS_ERR_OR_NULL(segs))
 		goto tg3_tso_bug_end;
 
 	do {
+		unsigned int desc_cnt = skb_shinfo(segs)->nr_frags + 1;
+
 		nskb = segs;
 		segs = segs->next;
 		nskb->next = NULL;
-		tg3_start_xmit(nskb, tp->dev);
+
+		if (tg3_tx_avail(tnapi) <= segs_remaining - 1 + desc_cnt &&
+		    skb_linearize(nskb)) {
+			nskb->next = segs;
+			segs = nskb;
+			do {
+				nskb = segs->next;
+
+				dev_kfree_skb_any(segs);
+				segs = nskb;
+			} while (segs);
+			goto tg3_tso_bug_end;
+		}
+		segs_remaining--;
+		if (segs_remaining)
+			__tg3_start_xmit(nskb, tp->dev, segs_remaining);
+		else
+			tg3_start_xmit(nskb, tp->dev);
 	} while (segs);
 
 tg3_tso_bug_end:
@@ -7877,6 +7904,12 @@ tg3_tso_bug_end:
 /* hard_start_xmit for all devices */
 static netdev_tx_t tg3_start_xmit(struct sk_buff *skb, struct net_device *dev)
 {
+	return __tg3_start_xmit(skb, dev, MAX_SKB_FRAGS + 1);
+}
+
+static netdev_tx_t __tg3_start_xmit(struct sk_buff *skb,
+				    struct net_device *dev, u32 stop_thresh)
+{
 	struct tg3 *tp = netdev_priv(dev);
 	u32 len, entry, base_flags, mss, vlan = 0;
 	u32 budget;
@@ -7905,12 +7938,17 @@ static netdev_tx_t tg3_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	if (unlikely(budget <= (skb_shinfo(skb)->nr_frags + 1))) {
 		if (!netif_tx_queue_stopped(txq)) {
 			netif_tx_stop_queue(txq);
+			tnapi->wakeup_thresh = TG3_TX_WAKEUP_THRESH(tnapi);
 
 			/* This is a hard error, log it. */
 			netdev_err(dev,
 				   "BUG! Tx Ring full when queue awake!\n");
 		}
-		return NETDEV_TX_BUSY;
+		smp_mb();
+		if (tg3_tx_avail(tnapi) <= tnapi->wakeup_thresh)
+			return NETDEV_TX_BUSY;
+
+		netif_tx_wake_queue(txq);
 	}
 
 	entry = tnapi->tx_prod;
@@ -8087,8 +8125,9 @@ static netdev_tx_t tg3_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	tw32_tx_mbox(tnapi->prodmbox, entry);
 
 	tnapi->tx_prod = entry;
-	if (unlikely(tg3_tx_avail(tnapi) <= (MAX_SKB_FRAGS + 1))) {
+	if (unlikely(tg3_tx_avail(tnapi) <= stop_thresh)) {
 		netif_tx_stop_queue(txq);
+		tnapi->wakeup_thresh = TG3_TX_WAKEUP_THRESH(tnapi);
 
 		/* netif_tx_stop_queue() must be done before checking
 		 * checking tx index in tg3_tx_avail() below, because in
@@ -8096,7 +8135,7 @@ static netdev_tx_t tg3_start_xmit(struct sk_buff *skb, struct net_device *dev)
 		 * netif_tx_queue_stopped().
 		 */
 		smp_mb();
-		if (tg3_tx_avail(tnapi) > TG3_TX_WAKEUP_THRESH(tnapi))
+		if (tg3_tx_avail(tnapi) > tnapi->wakeup_thresh)
 			netif_tx_wake_queue(txq);
 	}
 
@@ -12319,9 +12358,7 @@ static int tg3_set_ringparam(struct net_device *dev, struct ethtool_ringparam *e
 	if ((ering->rx_pending > tp->rx_std_ring_mask) ||
 	    (ering->rx_jumbo_pending > tp->rx_jmb_ring_mask) ||
 	    (ering->tx_pending > TG3_TX_RING_SIZE - 1) ||
-	    (ering->tx_pending <= MAX_SKB_FRAGS + 1) ||
-	    (tg3_flag(tp, TSO_BUG) &&
-	     (ering->tx_pending <= (MAX_SKB_FRAGS * 3))))
+	    (ering->tx_pending <= MAX_SKB_FRAGS + 1))
 		return -EINVAL;
 
 	if (netif_running(dev)) {
@@ -12341,6 +12378,7 @@ static int tg3_set_ringparam(struct net_device *dev, struct ethtool_ringparam *e
 	if (tg3_flag(tp, JUMBO_RING_ENABLE))
 		tp->rx_jumbo_pending = ering->rx_jumbo_pending;
 
+	dev->gso_max_segs = TG3_TX_SEG_PER_DESC(ering->tx_pending - 1);
 	for (i = 0; i < tp->irq_max; i++)
 		tp->napi[i].tx_pending = ering->tx_pending;
 
@@ -17817,6 +17855,7 @@ static int tg3_init_one(struct pci_dev *pdev,
 		else
 			sndmbx += 0xc;
 	}
+	dev->gso_max_segs = TG3_TX_SEG_PER_DESC(TG3_DEF_TX_RING_PENDING - 1);
 
 	tg3_init_coal(tp);
 
diff --git a/drivers/net/ethernet/broadcom/tg3.h b/drivers/net/ethernet/broadcom/tg3.h
index 461acca..6a7e13d 100644
--- a/drivers/net/ethernet/broadcom/tg3.h
+++ b/drivers/net/ethernet/broadcom/tg3.h
@@ -3006,6 +3006,7 @@ struct tg3_napi {
 	u32				tx_pending;
 	u32				last_tx_cons;
 	u32				prodmbox;
+	u32				wakeup_thresh;
 	struct tg3_tx_buffer_desc	*tx_ring;
 	struct tg3_tx_ring_info		*tx_buffers;
 
-- 
1.8.4.5

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