net: sched: fix packet stuck problem for lockless qdisc

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.

Also clear the STATE_MISSED before calling __netif_schedule()
at the end of qdisc_run_end() to avoid doing another round of
dequeuing in the pfifo_fast_dequeue().

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")
Acked-by: Jakub Kicinski <kuba@kernel.org>
Tested-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index f7a6e14..1e62551 100644 (file)
--- 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 +202,15 @@ 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))) {
+                       clear_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 (file)
--- 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);
        }