| Message ID | 1620266264-48109-1-git-send-email-linyunsheng@huawei.com |
|---|---|
| Headers | show |
| Series | fix packet stuck problem for lockless qdisc | expand |
On Thu, 6 May 2021 09:57:42 +0800 Yunsheng Lin wrote: > @@ -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 s/wil/will/ > + * the flag set after releasing lock and reschedule the > + * net_tx_action() to do the dequeuing. I don't understand why MISSED is checked before the trylock. Could you explain why it can't be tested directly here? > + */ > + 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)) { Why test_and_clear_bit() here? AFAICT this is the only place the bit is cleared. So the test and clear do not have to be atomic. To my limited understanding on x86 test_bit() is never a locked operation, while test_and_clear_bit() is always locked. So we'd save an atomic operation in un-contended case if we tested first and then cleared. > + /* do another dequeuing after clearing the flag to > + * avoid calling __netif_schedule(). > + */ > + smp_mb__after_atomic(); test_and_clear_bit() which returned true implies a memory barrier, AFAIU, so the barrier is not needed with the code as is. It will be needed if we switch to test_bit() + clear_bit(), but please clarify what it is paring with. > + need_retry = false; > + > + goto retry; > } else { > WRITE_ONCE(qdisc->empty, true); > }
On 2021/5/8 7:57, Jakub Kicinski wrote: > On Thu, 6 May 2021 09:57:42 +0800 Yunsheng Lin wrote: >> @@ -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 > > s/wil/will/ Thanks. > >> + * the flag set after releasing lock and reschedule the >> + * net_tx_action() to do the dequeuing. > > I don't understand why MISSED is checked before the trylock. > Could you explain why it can't be tested directly here? The initial thinking was: Just like the set_bit() before the second trylock, If MISSED is set before first trylock, it means other thread has set the MISSED flag for this thread before doing the first trylock, so that this thread does not need to do the set_bit(). But the initial thinking seems over thinking, as thread 3' setting the MISSED before the second trylock has ensure either thread 3' second trylock returns ture or thread 2 holding the lock will see the MISSED flag, so thread 1 can do the test_bit() before or after the first trylock, as below: thread 1 thread 2 thread 3 holding q->seqlock first trylock failed first trylock failed unlock q->seqlock test_bit(MISSED) return false test_bit(MISSED) return false and not reschedule set_bit(MISSED) trylock success test_bit(MISSED) retun ture and not retry second trylock If the above is correct, it seems we could: 1. do test_bit(MISSED) before the first trylock to avoid doing the first trylock for contended case. or 2. do test_bit(MISSED) after the first trylock to avoid doing the test_bit() for un-contended case. Which one do you prefer? > >> + */ >> + 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)) { > > Why test_and_clear_bit() here? AFAICT this is the only place the bit > is cleared. So the test and clear do not have to be atomic. The the bit is also cleared in other place in patch 2/3, but within the protection of q->seqlock too, so yes, the test and clear do not have to be atomic for performance sake. > > To my limited understanding on x86 test_bit() is never a locked > operation, while test_and_clear_bit() is always locked. So we'd save > an atomic operation in un-contended case if we tested first and then > cleared. > >> + /* do another dequeuing after clearing the flag to >> + * avoid calling __netif_schedule(). >> + */ >> + smp_mb__after_atomic(); > > test_and_clear_bit() which returned true implies a memory barrier, > AFAIU, so the barrier is not needed with the code as is. It will be > needed if we switch to test_bit() + clear_bit(), but please clarify > what it is paring with. Ok. Thanks for the reviewing. > >> + need_retry = false; >> + >> + goto retry; >> } else { >> WRITE_ONCE(qdisc->empty, true); >> } > > > . >
On Sat, 8 May 2021 10:55:19 +0800 Yunsheng Lin wrote: > >> + * the flag set after releasing lock and reschedule the > >> + * net_tx_action() to do the dequeuing. > > > > I don't understand why MISSED is checked before the trylock. > > Could you explain why it can't be tested directly here? > The initial thinking was: > Just like the set_bit() before the second trylock, If MISSED is set > before first trylock, it means other thread has set the MISSED flag > for this thread before doing the first trylock, so that this thread > does not need to do the set_bit(). > > But the initial thinking seems over thinking, as thread 3' setting the > MISSED before the second trylock has ensure either thread 3' second > trylock returns ture or thread 2 holding the lock will see the MISSED > flag, so thread 1 can do the test_bit() before or after the first > trylock, as below: > > thread 1 thread 2 thread 3 > holding q->seqlock > first trylock failed first trylock failed > unlock q->seqlock > test_bit(MISSED) return false > test_bit(MISSED) return false > and not reschedule > set_bit(MISSED) > trylock success > test_bit(MISSED) retun ture > and not retry second trylock > > If the above is correct, it seems we could: > 1. do test_bit(MISSED) before the first trylock to avoid doing the > first trylock for contended case. > or > 2. do test_bit(MISSED) after the first trylock to avoid doing the > test_bit() for un-contended case. > > Which one do you prefer? No strong preference but testing after the trylock seems more obvious as it saves the temporary variable. For the contended case could we potentially move or add a MISSED test before even the first try_lock()? I'm not good at optimizing things, but it could save us the atomic op, right? (at least on x86)
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. 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 | 37 ++++++++++++++++++++++++++++++++++++- net/core/dev.c | 30 +++++++++++++++++++++++++----- net/sched/sch_generic.c | 24 ++++++++++++++++++++++-- 4 files changed, 84 insertions(+), 14 deletions(-)