forked from mirrors/linux
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: 6b3ba9146f ("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>
This commit is contained in:
Yunsheng Lin
David S. Miller
parent
974271e5ed
commit
a90c57f2ce
2 changed files with 53 additions and 1 deletions
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@ -36,6 +36,7 @@ struct qdisc_rate_table {
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enum qdisc_state_t {
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__QDISC_STATE_SCHED,
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__QDISC_STATE_DEACTIVATED,
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__QDISC_STATE_MISSED,
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};
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struct qdisc_size_table {
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@ -159,8 +160,33 @@ static inline bool qdisc_is_empty(const struct Qdisc *qdisc)
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static inline bool qdisc_run_begin(struct Qdisc *qdisc)
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{
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if (qdisc->flags & TCQ_F_NOLOCK) {
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if (spin_trylock(&qdisc->seqlock))
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goto nolock_empty;
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/* If the MISSED flag is set, it means other thread has
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* set the MISSED flag before second spin_trylock(), so
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* we can return false here to avoid multi cpus doing
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* the set_bit() and second spin_trylock() concurrently.
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*/
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if (test_bit(__QDISC_STATE_MISSED, &qdisc->state))
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return false;
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/* Set the MISSED flag before the second spin_trylock(),
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* if the second spin_trylock() return false, it means
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* other cpu holding the lock will do dequeuing for us
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* or it will see the MISSED flag set after releasing
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* lock and reschedule the net_tx_action() to do the
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* dequeuing.
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*/
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set_bit(__QDISC_STATE_MISSED, &qdisc->state);
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/* Retry again in case other CPU may not see the new flag
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* after it releases the lock at the end of qdisc_run_end().
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*/
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if (!spin_trylock(&qdisc->seqlock))
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return false;
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nolock_empty:
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WRITE_ONCE(qdisc->empty, false);
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} else if (qdisc_is_running(qdisc)) {
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return false;
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@ -176,8 +202,15 @@ static inline bool qdisc_run_begin(struct Qdisc *qdisc)
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static inline void qdisc_run_end(struct Qdisc *qdisc)
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{
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write_seqcount_end(&qdisc->running);
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if (qdisc->flags & TCQ_F_NOLOCK)
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if (qdisc->flags & TCQ_F_NOLOCK) {
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spin_unlock(&qdisc->seqlock);
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if (unlikely(test_bit(__QDISC_STATE_MISSED,
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&qdisc->state))) {
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clear_bit(__QDISC_STATE_MISSED, &qdisc->state);
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__netif_schedule(qdisc);
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}
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}
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}
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static inline bool qdisc_may_bulk(const struct Qdisc *qdisc)
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@ -640,8 +640,10 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc)
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{
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struct pfifo_fast_priv *priv = qdisc_priv(qdisc);
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struct sk_buff *skb = NULL;
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bool need_retry = true;
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int band;
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retry:
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for (band = 0; band < PFIFO_FAST_BANDS && !skb; band++) {
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struct skb_array *q = band2list(priv, band);
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@ -652,6 +654,23 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc)
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}
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if (likely(skb)) {
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qdisc_update_stats_at_dequeue(qdisc, skb);
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} else if (need_retry &&
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test_bit(__QDISC_STATE_MISSED, &qdisc->state)) {
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/* Delay clearing the STATE_MISSED here to reduce
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* the overhead of the second spin_trylock() in
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* qdisc_run_begin() and __netif_schedule() calling
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* in qdisc_run_end().
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*/
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clear_bit(__QDISC_STATE_MISSED, &qdisc->state);
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/* Make sure dequeuing happens after clearing
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* STATE_MISSED.
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*/
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smp_mb__after_atomic();
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need_retry = false;
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goto retry;
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} else {
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WRITE_ONCE(qdisc->empty, true);
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}
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