[PATCH net v6 0/3] fix packet stuck problem for lockless qdisc

This patchset fixes the packet stuck problem mentioned in [1].

Patch 1: Add STATE_MISSED flag to fix packet stuck problem.
Patch 2: Fix a tx_action rescheduling problem after STATE_MISSED
         flag is added in patch 1.
Patch 3: Fix the significantly higher CPU consumption problem when
         multiple threads are competing on a saturated outgoing
         device.

V6: Some performance optimization in patch 1 suggested by Jakub
    and drop NET_XMIT_DROP checking in patch 3.
V5: add patch 3 to fix the problem reported by Michal Kubecek.
V4: Change STATE_NEED_RESCHEDULE to STATE_MISSED and add patch 2.

[1]. https://lkml.org/lkml/2019/10/9/42

Yunsheng Lin (3):
  net: sched: fix packet stuck problem for lockless qdisc
  net: sched: fix endless tx action reschedule during deactivation
  net: sched: fix tx action reschedule issue with stopped queue

 include/net/pkt_sched.h   |  7 +------
 include/net/sch_generic.h | 33 ++++++++++++++++++++++++++++++++-
 net/core/dev.c            | 29 ++++++++++++++++++++++++-----
 net/sched/sch_generic.c   | 31 +++++++++++++++++++++++++++++--
 4 files changed, 86 insertions(+), 14 deletions(-)

-- 
2.7.4

Currently qdisc_run() checks the STATE_DEACTIVATED of lockless
qdisc before calling __qdisc_run(), which ultimately clear the
STATE_MISSED when all the skb is dequeued. If STATE_DEACTIVATED
is set before clearing STATE_MISSED, there may be endless
rescheduling of net_tx_action() at the end of qdisc_run_end(),
see below:

CPU0(net_tx_atcion)  CPU1(__dev_xmit_skb)  CPU2(dev_deactivate)
          .                   .                     .
          .            set STATE_MISSED             .
          .           __netif_schedule()            .
          .                   .           set STATE_DEACTIVATED
          .                   .                qdisc_reset()
          .                   .                     .
          .<---------------   .              synchronize_net()
clear __QDISC_STATE_SCHED  |  .                     .
          .                |  .                     .
          .                |  .                     .
          .                |  .           --------->.
          .                |  .          |          .
  test STATE_DEACTIVATED   |  .          | some_qdisc_is_busy()
__qdisc_run() *not* called |  .          |-----return *true*
          .                |  .                     .
   test STATE_MISS         |  .                     .
 __netif_schedule()--------|  .                     .
          .                   .                     .
          .                   .                     .

__qdisc_run() is not called by net_tx_atcion() in CPU0 because
CPU2 has set STATE_DEACTIVATED flag during dev_deactivate(), and
STATE_MISSED is only cleared in __qdisc_run(), __netif_schedule
is called endlessly at the end of qdisc_run_end(), causing endless
tx action rescheduling problem.

qdisc_run() called by net_tx_action() runs in the softirq context,
which should has the same semantic as the qdisc_run() called by
__dev_xmit_skb() protected by rcu_read_lock_bh(). And there is a
synchronize_net() between STATE_DEACTIVATED flag being set and
qdisc_reset()/some_qdisc_is_busy in dev_deactivate(), we can safely
bail out for the deactived lockless qdisc in net_tx_action(), and
qdisc_reset() will reset all skb not dequeued yet.

So add the rcu_read_lock() explicitly to protect the qdisc_run()
and do the STATE_DEACTIVATED checking in net_tx_action() before
calling qdisc_run_begin(). Another option is to do the checking in
the qdisc_run_end(), but it will add unnecessary overhead for
non-tx_action case, because __dev_queue_xmit() will not see qdisc
with STATE_DEACTIVATED after synchronize_net(), the qdisc with
STATE_DEACTIVATED can only be seen by net_tx_action() because of
__netif_schedule().

The STATE_DEACTIVATED checking in qdisc_run() is to avoid race
between net_tx_action() and qdisc_reset(), see:
commit d518d2ed8640 ("net/sched: fix race between deactivation
and dequeue for NOLOCK qdisc"). As the bailout added above for
deactived lockless qdisc in net_tx_action() provides better
protection for the race without calling qdisc_run() at all, so
remove the STATE_DEACTIVATED checking in qdisc_run().

After qdisc_reset(), there is no skb in qdisc to be dequeued, so
clear the STATE_MISSED in dev_reset_queue() too.

Fixes: 6b3ba9146fe6 ("net: sched: allow qdiscs to handle locking")
Signed-off-by: Yunsheng Lin <redacted>
---
 include/net/pkt_sched.h |  7 +------
 net/core/dev.c          | 26 ++++++++++++++++++++++----
 net/sched/sch_generic.c |  4 +++-
 3 files changed, 26 insertions(+), 11 deletions(-)

diff --git a/include/net/pkt_sched.h b/include/net/pkt_sched.h
index f5c1bee..6d7b12c 100644
--- a/include/net/pkt_sched.h
+++ b/include/net/pkt_sched.h

@@ -128,12 +128,7 @@ void __qdisc_run(struct Qdisc *q);
 static inline void qdisc_run(struct Qdisc *q)
 {
 	if (qdisc_run_begin(q)) {
-		/* NOLOCK qdisc must check 'state' under the qdisc seqlock
-		 * to avoid racing with dev_qdisc_reset()
-		 */
-		if (!(q->flags & TCQ_F_NOLOCK) ||
-		    likely(!test_bit(__QDISC_STATE_DEACTIVATED, &q->state)))
-			__qdisc_run(q);
+		__qdisc_run(q);
 		qdisc_run_end(q);
 	}
 }

diff --git a/net/core/dev.c b/net/core/dev.c
index 222b1d3..d596cd7 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c

@@ -5025,25 +5025,43 @@ static __latent_entropy void net_tx_action(struct softirq_action *h)
 		sd->output_queue_tailp = &sd->output_queue;
 		local_irq_enable();
 
+		rcu_read_lock();
+
 		while (head) {
 			struct Qdisc *q = head;
 			spinlock_t *root_lock = NULL;
 
 			head = head->next_sched;
 
-			if (!(q->flags & TCQ_F_NOLOCK)) {
-				root_lock = qdisc_lock(q);
-				spin_lock(root_lock);
-			}
 			/* We need to make sure head->next_sched is read
 			 * before clearing __QDISC_STATE_SCHED
 			 */
 			smp_mb__before_atomic();
+
+			if (!(q->flags & TCQ_F_NOLOCK)) {
+				root_lock = qdisc_lock(q);
+				spin_lock(root_lock);
+			} else if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED,
+						     &q->state))) {
+				/* There is a synchronize_net() between
+				 * STATE_DEACTIVATED flag being set and
+				 * qdisc_reset()/some_qdisc_is_busy() in
+				 * dev_deactivate(), so we can safely bail out
+				 * early here to avoid data race between
+				 * qdisc_deactivate() and some_qdisc_is_busy()
+				 * for lockless qdisc.
+				 */
+				clear_bit(__QDISC_STATE_SCHED, &q->state);
+				continue;
+			}
+
 			clear_bit(__QDISC_STATE_SCHED, &q->state);
 			qdisc_run(q);
 			if (root_lock)
 				spin_unlock(root_lock);
 		}
+
+		rcu_read_unlock();
 	}
 
 	xfrm_dev_backlog(sd);

diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 795d986..d86c4cc 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c

@@ -1177,8 +1177,10 @@ static void dev_reset_queue(struct net_device *dev,
 	qdisc_reset(qdisc);
 
 	spin_unlock_bh(qdisc_lock(qdisc));
-	if (nolock)
+	if (nolock) {
+		clear_bit(__QDISC_STATE_MISSED, &qdisc->state);
 		spin_unlock_bh(&qdisc->seqlock);
+	}
 }
 
 static bool some_qdisc_is_busy(struct net_device *dev)

-- 
2.7.4

Lockless qdisc has below concurrent problem:
    cpu0                 cpu1
     .                     .
q->enqueue                 .
     .                     .
qdisc_run_begin()          .
     .                     .
dequeue_skb()              .
     .                     .
sch_direct_xmit()          .
     .                     .
     .                q->enqueue
     .             qdisc_run_begin()
     .            return and do nothing
     .                     .
qdisc_run_end()            .

cpu1 enqueue a skb without calling __qdisc_run() because cpu0
has not released the lock yet and spin_trylock() return false
for cpu1 in qdisc_run_begin(), and cpu0 do not see the skb
enqueued by cpu1 when calling dequeue_skb() because cpu1 may
enqueue the skb after cpu0 calling dequeue_skb() and before
cpu0 calling qdisc_run_end().

Lockless qdisc has below another concurrent problem when
tx_action is involved:

cpu0(serving tx_action)     cpu1             cpu2
          .                   .                .
          .              q->enqueue            .
          .            qdisc_run_begin()       .
          .              dequeue_skb()         .
          .                   .            q->enqueue
          .                   .                .
          .             sch_direct_xmit()      .
          .                   .         qdisc_run_begin()
          .                   .       return and do nothing
          .                   .                .
 clear __QDISC_STATE_SCHED    .                .
 qdisc_run_begin()            .                .
 return and do nothing        .                .
          .                   .                .
          .            qdisc_run_end()         .

This patch fixes the above data race by:
1. If the first spin_trylock() return false and STATE_MISSED is
   not set, set STATE_MISSED and retry another spin_trylock() in
   case other CPU may not see STATE_MISSED after it releases the
   lock.
2. reschedule if STATE_MISSED is set after the lock is released
   at the end of qdisc_run_end().

For tx_action case, STATE_MISSED is also set when cpu1 is at the
end if qdisc_run_end(), so tx_action will be rescheduled again
to dequeue the skb enqueued by cpu2.

Clear STATE_MISSED before retrying a dequeuing when dequeuing
returns NULL in order to reduce the overhead of the second
spin_trylock() and __netif_schedule() calling.

The performance impact of this patch, tested using pktgen and
dummy netdev with pfifo_fast qdisc attached:

 threads  without+this_patch   with+this_patch      delta
    1        2.61Mpps            2.60Mpps           -0.3%
    2        3.97Mpps            3.82Mpps           -3.7%
    4        5.62Mpps            5.59Mpps           -0.5%
    8        2.78Mpps            2.77Mpps           -0.3%
   16        2.22Mpps            2.22Mpps           -0.0%

Fixes: 6b3ba9146fe6 ("net: sched: allow qdiscs to handle locking")
Signed-off-by: Yunsheng Lin <redacted>
Tested-by: Juergen Gross <jgross@suse.com>
---
V6: Check MISSED after the first trylock, and remove the
    automic test and set for performance sake as suggested
    by Jakub.
V4: Change STATE_NEED_RESCHEDULE to STATE_MISSED mirroring
    NAPI's NAPIF_STATE_MISSED, and add Juergen's "Tested-by"
    tag for there is only renaming and typo fixing between
    V4 and V3.
V3: Fix a compile error and a few comment typo, remove the
    __QDISC_STATE_DEACTIVATED checking, and update the
    performance data.
V2: Avoid the overhead of fixing the data race as much as
    possible.
---
 include/net/sch_generic.h | 33 ++++++++++++++++++++++++++++++++-
 net/sched/sch_generic.c   | 19 +++++++++++++++++++
 2 files changed, 51 insertions(+), 1 deletion(-)

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index f7a6e14..e5df394 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h

@@ -36,6 +36,7 @@ struct qdisc_rate_table {
 enum qdisc_state_t {
 	__QDISC_STATE_SCHED,
 	__QDISC_STATE_DEACTIVATED,
+	__QDISC_STATE_MISSED,
 };
 
 struct qdisc_size_table {

@@ -159,8 +160,33 @@ static inline bool qdisc_is_empty(const struct Qdisc *qdisc)
 static inline bool qdisc_run_begin(struct Qdisc *qdisc)
 {
 	if (qdisc->flags & TCQ_F_NOLOCK) {
+		if (spin_trylock(&qdisc->seqlock))
+			goto nolock_empty;
+
+		/* If the MISSED flag is set, it means other thread has
+		 * set the MISSED flag before second spin_trylock(), so
+		 * we can return false here to avoid multi cpus doing
+		 * the set_bit() and second spin_trylock() concurrently.
+		 */
+		if (test_bit(__QDISC_STATE_MISSED, &qdisc->state))
+			return false;
+
+		/* Set the MISSED flag before the second spin_trylock(),
+		 * if the second spin_trylock() return false, it means
+		 * other cpu holding the lock will do dequeuing for us
+		 * or it will see the MISSED flag set after releasing
+		 * lock and reschedule the net_tx_action() to do the
+		 * dequeuing.
+		 */
+		set_bit(__QDISC_STATE_MISSED, &qdisc->state);
+
+		/* Retry again in case other CPU may not see the new flag
+		 * after it releases the lock at the end of qdisc_run_end().
+		 */
 		if (!spin_trylock(&qdisc->seqlock))
 			return false;
+
+nolock_empty:
 		WRITE_ONCE(qdisc->empty, false);
 	} else if (qdisc_is_running(qdisc)) {
 		return false;

@@ -176,8 +202,13 @@ static inline bool qdisc_run_begin(struct Qdisc *qdisc)
 static inline void qdisc_run_end(struct Qdisc *qdisc)
 {
 	write_seqcount_end(&qdisc->running);
-	if (qdisc->flags & TCQ_F_NOLOCK)
+	if (qdisc->flags & TCQ_F_NOLOCK) {
 		spin_unlock(&qdisc->seqlock);
+
+		if (unlikely(test_bit(__QDISC_STATE_MISSED,
+				      &qdisc->state)))
+			__netif_schedule(qdisc);
+	}
 }
 
 static inline bool qdisc_may_bulk(const struct Qdisc *qdisc)

diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 44991ea..795d986 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c

@@ -640,8 +640,10 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc)
 {
 	struct pfifo_fast_priv *priv = qdisc_priv(qdisc);
 	struct sk_buff *skb = NULL;
+	bool need_retry = true;
 	int band;
 
+retry:
 	for (band = 0; band < PFIFO_FAST_BANDS && !skb; band++) {
 		struct skb_array *q = band2list(priv, band);
 

@@ -652,6 +654,23 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc)
 	}
 	if (likely(skb)) {
 		qdisc_update_stats_at_dequeue(qdisc, skb);
+	} else if (need_retry &&
+		   test_bit(__QDISC_STATE_MISSED, &qdisc->state)) {
+		/* Delay clearing the STATE_MISSED here to reduce
+		 * the overhead of the second spin_trylock() in
+		 * qdisc_run_begin() and __netif_schedule() calling
+		 * in qdisc_run_end().
+		 */
+		clear_bit(__QDISC_STATE_MISSED, &qdisc->state);
+
+		/* Make sure dequeuing happens after clearing
+		 * STATE_MISSED.
+		 */
+		smp_mb__after_atomic();
+
+		need_retry = false;
+
+		goto retry;
 	} else {
 		WRITE_ONCE(qdisc->empty, true);
 	}

-- 
2.7.4

The netdev qeueue might be stopped when byte queue limit has
reached or tx hw ring is full, net_tx_action() may still be
rescheduled endlessly if STATE_MISSED is set, which consumes
a lot of cpu without dequeuing and transmiting any skb because
the netdev queue is stopped, see qdisc_run_end().

This patch fixes it by checking the netdev queue state before
calling qdisc_run() and clearing STATE_MISSED if netdev queue is
stopped during qdisc_run(), the net_tx_action() is recheduled
again when netdev qeueue is restarted, see netif_tx_wake_queue().

Fixes: 6b3ba9146fe6 ("net: sched: allow qdiscs to handle locking")
Reported-by: Michal Kubecek <redacted>
Signed-off-by: Yunsheng Lin <redacted>
---
V6: Drop NET_XMIT_DROP checking for it is not really relevant
    to this patch, and it may cause performance performance
    regression with multi pktgen threads on dummy netdev with
    pfifo_fast qdisc case.
---
 net/core/dev.c          | 3 ++-
 net/sched/sch_generic.c | 8 +++++++-
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index d596cd7..ef8cf76 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c

@@ -3853,7 +3853,8 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
 
 	if (q->flags & TCQ_F_NOLOCK) {
 		rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK;
-		qdisc_run(q);
+		if (likely(!netif_xmit_frozen_or_stopped(txq)))
+			qdisc_run(q);
 
 		if (unlikely(to_free))
 			kfree_skb_list(to_free);

diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index d86c4cc..37403c1 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c

@@ -74,6 +74,7 @@ static inline struct sk_buff *__skb_dequeue_bad_txq(struct Qdisc *q)
 			}
 		} else {
 			skb = SKB_XOFF_MAGIC;
+			clear_bit(__QDISC_STATE_MISSED, &q->state);
 		}
 	}
 

@@ -242,6 +243,7 @@ static struct sk_buff *dequeue_skb(struct Qdisc *q, bool *validate,
 			}
 		} else {
 			skb = NULL;
+			clear_bit(__QDISC_STATE_MISSED, &q->state);
 		}
 		if (lock)
 			spin_unlock(lock);

@@ -251,8 +253,10 @@ static struct sk_buff *dequeue_skb(struct Qdisc *q, bool *validate,
 	*validate = true;
 
 	if ((q->flags & TCQ_F_ONETXQUEUE) &&
-	    netif_xmit_frozen_or_stopped(txq))
+	    netif_xmit_frozen_or_stopped(txq)) {
+		clear_bit(__QDISC_STATE_MISSED, &q->state);
 		return skb;
+	}
 
 	skb = qdisc_dequeue_skb_bad_txq(q);
 	if (unlikely(skb)) {

@@ -311,6 +315,8 @@ bool sch_direct_xmit(struct sk_buff *skb, struct Qdisc *q,
 		HARD_TX_LOCK(dev, txq, smp_processor_id());
 		if (!netif_xmit_frozen_or_stopped(txq))
 			skb = dev_hard_start_xmit(skb, dev, txq, &ret);
+		else
+			clear_bit(__QDISC_STATE_MISSED, &q->state);
 
 		HARD_TX_UNLOCK(dev, txq);
 	} else {

-- 
2.7.4

On Mon, 10 May 2021 09:42:36 +0800 Yunsheng Lin wrote:

The netdev qeueue might be stopped when byte queue limit has
reached or tx hw ring is full, net_tx_action() may still be
rescheduled endlessly if STATE_MISSED is set, which consumes
a lot of cpu without dequeuing and transmiting any skb because
the netdev queue is stopped, see qdisc_run_end().

This patch fixes it by checking the netdev queue state before
calling qdisc_run() and clearing STATE_MISSED if netdev queue is
stopped during qdisc_run(), the net_tx_action() is recheduled
again when netdev qeueue is restarted, see netif_tx_wake_queue().

Fixes: 6b3ba9146fe6 ("net: sched: allow qdiscs to handle locking")
Reported-by: Michal Kubecek <redacted>
Signed-off-by: Yunsheng Lin <redacted>

Patches 1 and 2 look good to me but this one I'm not 100% sure.

The queues are woken asynchronously without holding any locks via
netif_tx_wake_queue(). Theoretically we can have a situation where:

CPU 0                            CPU 1   
  .                                .
dequeue_skb()                      .
  netif_xmit_frozen..() # true     .
  .                              [IRQ]
  .                              netif_tx_wake_queue()
  .                              <end of IRQ>
  .                              netif_tx_action()
  .                              set MISSED
  clear MISSED
  return NULL
ret from qdisc_restart()
ret from __qdisc_run()
qdisc_run_end()
-> MISSED not set

On 2021/5/11 12:22, Jakub Kicinski wrote:

On Mon, 10 May 2021 09:42:36 +0800 Yunsheng Lin wrote:

quoted

The netdev qeueue might be stopped when byte queue limit has
reached or tx hw ring is full, net_tx_action() may still be
rescheduled endlessly if STATE_MISSED is set, which consumes
a lot of cpu without dequeuing and transmiting any skb because
the netdev queue is stopped, see qdisc_run_end().

This patch fixes it by checking the netdev queue state before
calling qdisc_run() and clearing STATE_MISSED if netdev queue is
stopped during qdisc_run(), the net_tx_action() is recheduled
again when netdev qeueue is restarted, see netif_tx_wake_queue().

Fixes: 6b3ba9146fe6 ("net: sched: allow qdiscs to handle locking")
Reported-by: Michal Kubecek <redacted>
Signed-off-by: Yunsheng Lin <redacted>

Patches 1 and 2 look good to me but this one I'm not 100% sure.

quoted

@@ -251,8 +253,10 @@ static struct sk_buff *dequeue_skb(struct Qdisc *q, bool *validate,
 	*validate = true;
 
 	if ((q->flags & TCQ_F_ONETXQUEUE) &&
-	    netif_xmit_frozen_or_stopped(txq))
+	    netif_xmit_frozen_or_stopped(txq)) {
+		clear_bit(__QDISC_STATE_MISSED, &q->state);
 		return skb;
+	}

The queues are woken asynchronously without holding any locks via
netif_tx_wake_queue(). Theoretically we can have a situation where:

CPU 0                            CPU 1   
  .                                .
dequeue_skb()                      .
  netif_xmit_frozen..() # true     .
  .                              [IRQ]
  .                              netif_tx_wake_queue()
  .                              <end of IRQ>
  .                              netif_tx_action()
  .                              set MISSED
  clear MISSED
  return NULL
ret from qdisc_restart()
ret from __qdisc_run()
qdisc_run_end()
-> MISSED not set

Yes, the above does seems to have the above data race.

As my understanding, there is two ways to fix the above data race:
1. do not clear the STATE_MISSED for netif_xmit_frozen_or_stopped()
   case, just check the netif_xmit_frozen_or_stopped() before
   calling __netif_schedule() at the end of qdisc_run_end(). This seems
   to only work with qdisc with TCQ_F_ONETXQUEUE flag because it seems
   we can only check the netif_xmit_frozen_or_stopped() with q->dev_queue,
   I am not sure q->dev_queue is pointint to which netdev queue when qdisc
   is not set with TCQ_F_ONETXQUEUE flag.

2. clearing the STATE_MISSED for netif_xmit_frozen_or_stopped() case
   as this patch does, and protect the __netif_schedule() with q->seqlock
   for netif_tx_wake_queue(), which might bring unnecessary overhead for
   non-stopped queue case

Any better idea?

.

On 2021/5/11 17:04, Yunsheng Lin wrote:

On 2021/5/11 12:22, Jakub Kicinski wrote:

quoted

On Mon, 10 May 2021 09:42:36 +0800 Yunsheng Lin wrote:

quoted

The netdev qeueue might be stopped when byte queue limit has
reached or tx hw ring is full, net_tx_action() may still be
rescheduled endlessly if STATE_MISSED is set, which consumes
a lot of cpu without dequeuing and transmiting any skb because
the netdev queue is stopped, see qdisc_run_end().

This patch fixes it by checking the netdev queue state before
calling qdisc_run() and clearing STATE_MISSED if netdev queue is
stopped during qdisc_run(), the net_tx_action() is recheduled
again when netdev qeueue is restarted, see netif_tx_wake_queue().

Fixes: 6b3ba9146fe6 ("net: sched: allow qdiscs to handle locking")
Reported-by: Michal Kubecek <redacted>
Signed-off-by: Yunsheng Lin <redacted>

Patches 1 and 2 look good to me but this one I'm not 100% sure.

quoted

@@ -251,8 +253,10 @@ static struct sk_buff *dequeue_skb(struct Qdisc *q, bool *validate,
 	*validate = true;
 
 	if ((q->flags & TCQ_F_ONETXQUEUE) &&
-	    netif_xmit_frozen_or_stopped(txq))
+	    netif_xmit_frozen_or_stopped(txq)) {
+		clear_bit(__QDISC_STATE_MISSED, &q->state);
 		return skb;
+	}

The queues are woken asynchronously without holding any locks via
netif_tx_wake_queue(). Theoretically we can have a situation where:

CPU 0                            CPU 1   
  .                                .
dequeue_skb()                      .
  netif_xmit_frozen..() # true     .
  .                              [IRQ]
  .                              netif_tx_wake_queue()
  .                              <end of IRQ>
  .                              netif_tx_action()
  .                              set MISSED
  clear MISSED
  return NULL
ret from qdisc_restart()
ret from __qdisc_run()
qdisc_run_end()
-> MISSED not set

Yes, the above does seems to have the above data race.

As my understanding, there is two ways to fix the above data race:
1. do not clear the STATE_MISSED for netif_xmit_frozen_or_stopped()
   case, just check the netif_xmit_frozen_or_stopped() before
   calling __netif_schedule() at the end of qdisc_run_end(). This seems
   to only work with qdisc with TCQ_F_ONETXQUEUE flag because it seems
   we can only check the netif_xmit_frozen_or_stopped() with q->dev_queue,
   I am not sure q->dev_queue is pointint to which netdev queue when qdisc
   is not set with TCQ_F_ONETXQUEUE flag.

2. clearing the STATE_MISSED for netif_xmit_frozen_or_stopped() case
   as this patch does, and protect the __netif_schedule() with q->seqlock
   for netif_tx_wake_queue(), which might bring unnecessary overhead for
   non-stopped queue case

Any better idea?

3. Or check the netif_xmit_frozen_or_stopped() again after clearing
   STATE_MISSED, like below:

   if (netif_xmit_frozen_or_stopped(txq)) {
	  clear_bit(__QDISC_STATE_MISSED, &q->state);

	  /* Make sure the below netif_xmit_frozen_or_stopped()
	   * checking happens after clearing STATE_MISSED.
	   */
	  smp_mb__after_atomic();

	  /* Checking netif_xmit_frozen_or_stopped() again to
	   * make sure __QDISC_STATE_MISSED is set if the
	   * __QDISC_STATE_MISSED set by netif_tx_wake_queue()'s
	   * rescheduling of net_tx_action() is cleared by the
	   * above clear_bit().
	   */
	  if (!netif_xmit_frozen_or_stopped(txq))
	  	set_bit(__QDISC_STATE_MISSED, &q->state);
  }

  It is kind of ugly, but it does seem to fix the above data race too.
  And it seems like a common pattern to deal with the concurrency between
  xmit and NAPI polling, as below:

https://elixir.bootlin.com/linux/v5.12-rc2/source/drivers/net/ethernet/hisilicon/hns3/hns3_enet.c#L1409

quoted

.

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On Tue, 11 May 2021 20:13:56 +0800 Yunsheng Lin wrote:

On 2021/5/11 17:04, Yunsheng Lin wrote:

quoted

On 2021/5/11 12:22, Jakub Kicinski wrote:  

quoted

The queues are woken asynchronously without holding any locks via
netif_tx_wake_queue(). Theoretically we can have a situation where:

CPU 0                            CPU 1   
  .                                .
dequeue_skb()                      .
  netif_xmit_frozen..() # true     .
  .                              [IRQ]
  .                              netif_tx_wake_queue()
  .                              <end of IRQ>
  .                              netif_tx_action()
  .                              set MISSED
  clear MISSED
  return NULL
ret from qdisc_restart()
ret from __qdisc_run()
qdisc_run_end()  

 [...]  

quoted

Yes, the above does seems to have the above data race.

As my understanding, there is two ways to fix the above data race:
1. do not clear the STATE_MISSED for netif_xmit_frozen_or_stopped()
   case, just check the netif_xmit_frozen_or_stopped() before
   calling __netif_schedule() at the end of qdisc_run_end(). This seems
   to only work with qdisc with TCQ_F_ONETXQUEUE flag because it seems
   we can only check the netif_xmit_frozen_or_stopped() with q->dev_queue,
   I am not sure q->dev_queue is pointint to which netdev queue when qdisc
   is not set with TCQ_F_ONETXQUEUE flag.

Isn't the case where we have a NOLOCK qdisc without TCQ_F_ONETXQUEUE
rather unexpected? It'd have to be a single pfifo on multi-queue
netdev, right? Sounds not worth optimizing for. How about:

 static inline void qdisc_run_end(struct Qdisc *qdisc)
 {
 	write_seqcount_end(&qdisc->running);
	if (qdisc->flags & TCQ_F_NOLOCK) {
 		spin_unlock(&qdisc->seqlock);

		if (unlikely(test_bit(__QDISC_STATE_MISSED,
				      &qdisc->state))) {
			clear_bit(__QDISC_STATE_MISSED, &qdisc->state);
			if (!(q->flags & TCQ_F_ONETXQUEUE) ||
			    !netif_xmit_frozen_or_stopped(q->dev_queue))
				__netif_schedule(qdisc);
		}
	}
 }

For the strange non-ONETXQUEUE case we'd have an occasional unnecessary
net_tx_action, but no infinite loop possible.

quoted

2. clearing the STATE_MISSED for netif_xmit_frozen_or_stopped() case
   as this patch does, and protect the __netif_schedule() with q->seqlock
   for netif_tx_wake_queue(), which might bring unnecessary overhead for
   non-stopped queue case

Any better idea?  

3. Or check the netif_xmit_frozen_or_stopped() again after clearing
   STATE_MISSED, like below:

   if (netif_xmit_frozen_or_stopped(txq)) {
	  clear_bit(__QDISC_STATE_MISSED, &q->state);

	  /* Make sure the below netif_xmit_frozen_or_stopped()
	   * checking happens after clearing STATE_MISSED.
	   */
	  smp_mb__after_atomic();

	  /* Checking netif_xmit_frozen_or_stopped() again to
	   * make sure __QDISC_STATE_MISSED is set if the
	   * __QDISC_STATE_MISSED set by netif_tx_wake_queue()'s
	   * rescheduling of net_tx_action() is cleared by the
	   * above clear_bit().
	   */
	  if (!netif_xmit_frozen_or_stopped(txq))
	  	set_bit(__QDISC_STATE_MISSED, &q->state);
  }

  It is kind of ugly, but it does seem to fix the above data race too.
  And it seems like a common pattern to deal with the concurrency between
  xmit and NAPI polling, as below:

https://elixir.bootlin.com/linux/v5.12-rc2/source/drivers/net/ethernet/hisilicon/hns3/hns3_enet.c#L1409

This is indeed the idiomatic way of dealing with Tx queue stopping race,
but it's a bit of code to sprinkle around. My vote would be option 1.

On 2021/5/12 7:30, Jakub Kicinski wrote:

On Tue, 11 May 2021 20:13:56 +0800 Yunsheng Lin wrote:

quoted

On 2021/5/11 17:04, Yunsheng Lin wrote:

quoted

On 2021/5/11 12:22, Jakub Kicinski wrote:  

quoted

The queues are woken asynchronously without holding any locks via
netif_tx_wake_queue(). Theoretically we can have a situation where:

CPU 0                            CPU 1   
  .                                .
dequeue_skb()                      .
  netif_xmit_frozen..() # true     .
  .                              [IRQ]
  .                              netif_tx_wake_queue()
  .                              <end of IRQ>
  .                              netif_tx_action()
  .                              set MISSED
  clear MISSED
  return NULL
ret from qdisc_restart()
ret from __qdisc_run()
qdisc_run_end()  

 [...]  

quoted

Yes, the above does seems to have the above data race.

As my understanding, there is two ways to fix the above data race:
1. do not clear the STATE_MISSED for netif_xmit_frozen_or_stopped()
   case, just check the netif_xmit_frozen_or_stopped() before
   calling __netif_schedule() at the end of qdisc_run_end(). This seems
   to only work with qdisc with TCQ_F_ONETXQUEUE flag because it seems
   we can only check the netif_xmit_frozen_or_stopped() with q->dev_queue,
   I am not sure q->dev_queue is pointint to which netdev queue when qdisc
   is not set with TCQ_F_ONETXQUEUE flag.

Isn't the case where we have a NOLOCK qdisc without TCQ_F_ONETXQUEUE
rather unexpected? It'd have to be a single pfifo on multi-queue
netdev, right? Sounds not worth optimizing for. How about:

 static inline void qdisc_run_end(struct Qdisc *qdisc)
 {
 	write_seqcount_end(&qdisc->running);
	if (qdisc->flags & TCQ_F_NOLOCK) {
 		spin_unlock(&qdisc->seqlock);

		if (unlikely(test_bit(__QDISC_STATE_MISSED,
				      &qdisc->state))) {
			clear_bit(__QDISC_STATE_MISSED, &qdisc->state);
			if (!(q->flags & TCQ_F_ONETXQUEUE) ||
			    !netif_xmit_frozen_or_stopped(q->dev_queue))
				__netif_schedule(qdisc);
		}
	}
 }

For the strange non-ONETXQUEUE case we'd have an occasional unnecessary
net_tx_action, but no infinite loop possible.

quoted

quoted

2. clearing the STATE_MISSED for netif_xmit_frozen_or_stopped() case
   as this patch does, and protect the __netif_schedule() with q->seqlock
   for netif_tx_wake_queue(), which might bring unnecessary overhead for
   non-stopped queue case

Any better idea?  

3. Or check the netif_xmit_frozen_or_stopped() again after clearing
   STATE_MISSED, like below:

   if (netif_xmit_frozen_or_stopped(txq)) {
	  clear_bit(__QDISC_STATE_MISSED, &q->state);

	  /* Make sure the below netif_xmit_frozen_or_stopped()
	   * checking happens after clearing STATE_MISSED.
	   */
	  smp_mb__after_atomic();

	  /* Checking netif_xmit_frozen_or_stopped() again to
	   * make sure __QDISC_STATE_MISSED is set if the
	   * __QDISC_STATE_MISSED set by netif_tx_wake_queue()'s
	   * rescheduling of net_tx_action() is cleared by the
	   * above clear_bit().
	   */
	  if (!netif_xmit_frozen_or_stopped(txq))
	  	set_bit(__QDISC_STATE_MISSED, &q->state);
  }

  It is kind of ugly, but it does seem to fix the above data race too.
  And it seems like a common pattern to deal with the concurrency between
  xmit and NAPI polling, as below:

https://elixir.bootlin.com/linux/v5.12-rc2/source/drivers/net/ethernet/hisilicon/hns3/hns3_enet.c#L1409

This is indeed the idiomatic way of dealing with Tx queue stopping race,
but it's a bit of code to sprinkle around. My vote would be option 1.

I had done some performance testing to see which is better, tested using
pktgen and dummy netdev with pfifo_fast qdisc attached:

unit: Mpps

threads    V6         V6 + option 1     V6 + option 3
  1       2.60          2.54               2.60
  2       3.86          3.84               3.84
  4       5.56          5.50               5.51
  8       2.79          2.77               2.77
  16      2.23          2.24               2.22

So it seems the netif_xmit_frozen_or_stopped checking overhead for non-stopped queue
is noticable for 1 pktgen thread.

And the performance increase for V6 + option 1 with 16 pktgen threads is because of
"clear_bit(__QDISC_STATE_MISSED, &qdisc->state)" at the end of qdisc_run_end(), which
may avoid the another round of dequeuing in the pfifo_fast_dequeue(). And adding the
"clear_bit(__QDISC_STATE_MISSED, &qdisc->state)"  for V6 + option 3, the data for
16 pktgen thread also go up to 2.24Mpps.


So it seems V6 + option 3 with "clear_bit(__QDISC_STATE_MISSED, &qdisc->state)" at
the end of qdisc_run_end() is better?

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On Wed, 12 May 2021 11:34:55 +0800 Yunsheng Lin wrote:

quoted

This is indeed the idiomatic way of dealing with Tx queue stopping race,
but it's a bit of code to sprinkle around. My vote would be option 1.  

I had done some performance testing to see which is better, tested using
pktgen and dummy netdev with pfifo_fast qdisc attached:

unit: Mpps

threads    V6         V6 + option 1     V6 + option 3
  1       2.60          2.54               2.60
  2       3.86          3.84               3.84
  4       5.56          5.50               5.51
  8       2.79          2.77               2.77
  16      2.23          2.24               2.22

So it seems the netif_xmit_frozen_or_stopped checking overhead for non-stopped queue
is noticable for 1 pktgen thread.

And the performance increase for V6 + option 1 with 16 pktgen threads is because of
"clear_bit(__QDISC_STATE_MISSED, &qdisc->state)" at the end of qdisc_run_end(), which
may avoid the another round of dequeuing in the pfifo_fast_dequeue(). And adding the
"clear_bit(__QDISC_STATE_MISSED, &qdisc->state)"  for V6 + option 3, the data for
16 pktgen thread also go up to 2.24Mpps.


So it seems V6 + option 3 with "clear_bit(__QDISC_STATE_MISSED, &qdisc->state)" at
the end of qdisc_run_end() is better?

Alright, sounds good.