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. Test STATE_MISSED before doing spin_trylock(). 2. If the first spin_trylock() return false and STATE_MISSED is not set before the first spin_trylock(), Set STATE_MISSED and retry another spin_trylock() in case other CPU may not see STATE_MISSED after it releases the lock. 3. 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 above double 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 linyunsheng@huawei.com Tested-by: Juergen Gross jgross@suse.com --- 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 | 37 ++++++++++++++++++++++++++++++++++++- net/sched/sch_generic.c | 12 ++++++++++++ 2 files changed, 48 insertions(+), 1 deletion(-)
diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h index f7a6e14..b85b8ea 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 +160,37 @@ 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) { + bool dont_retry = test_bit(__QDISC_STATE_MISSED, + &qdisc->state); + + if (spin_trylock(&qdisc->seqlock)) + goto nolock_empty; + + /* If the flag is set before doing the spin_trylock() and + * the above spin_trylock() return false, it means other cpu + * holding the lock will do dequeuing for us, or it wil see + * the flag set after releasing lock and reschedule the + * net_tx_action() to do the dequeuing. + */ + if (dont_retry) + return false; + + /* We could do set_bit() before the first spin_trylock(), + * and avoid doing second spin_trylock() completely, then + * we could have multi cpus doing the set_bit(). Here use + * dont_retry to avoid doing the set_bit() and the second + * spin_trylock(), which has 5% performance improvement than + * doing the set_bit() before the first spin_trylock(). + */ + 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 +206,13 @@ 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..9bc73ea 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 +654,16 @@ 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_and_clear_bit(__QDISC_STATE_MISSED, + &qdisc->state)) { + /* do another dequeuing after clearing the flag to + * avoid calling __netif_schedule(). + */ + smp_mb__after_atomic(); + need_retry = false; + + goto retry; } else { WRITE_ONCE(qdisc->empty, true); }