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From:   Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>
To:     Yibo Zhao <yiboz@codeaurora.org>
Cc:     ath10k@lists.infradead.org, linux-wireless@vger.kernel.org,
        linux-wireless-owner@vger.kernel.org
Subject: Re: [PATCH 2/4] mac80211: defer txqs removal from rbtree
In-Reply-To: <910d9bb5f9016b29fb2aaeb0b89bac38@codeaurora.org>
References: <1568639388-27291-1-git-send-email-yiboz@codeaurora.org> <1568639388-27291-2-git-send-email-yiboz@codeaurora.org> <87pnjyiq7o.fsf@toke.dk> <c3ee7ece0986f1c50513cd5fdd2ee03f@codeaurora.org> <87sgothmpy.fsf@toke.dk> <8cdece5c030fd95817fb099021c38613@codeaurora.org> <87tv98fu6l.fsf@toke.dk> <1b4ab006d9b5c88035845aaac193ef48@codeaurora.org> <8736gre3bm.fsf@toke.dk> <198124204167325252fcfcd65e3f2733@codeaurora.org> <87ftkp7uuz.fsf@toke.dk> <4574cce4079f8dab2b2bf223431a6eae@codeaurora.org> <877e617qg2.fsf@toke.dk> <910d9bb5f9016b29fb2aaeb0b89bac38@codeaurora.org>
Date:   Sat, 21 Sep 2019 16:00:06 +0200
Message-ID: <874l157nrt.fsf@toke.dk>
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Yibo Zhao <yiboz@codeaurora.org> writes:

> On 2019-09-21 21:02, Toke H=C3=B8iland-J=C3=B8rgensen wrote:
>> Yibo Zhao <yiboz@codeaurora.org> writes:
>>=20
>>> On 2019-09-21 19:27, Toke H=C3=B8iland-J=C3=B8rgensen wrote:
>>>> Yibo Zhao <yiboz@codeaurora.org> writes:
>>>>=20
>>>>> On 2019-09-20 17:15, Toke H=C3=B8iland-J=C3=B8rgensen wrote:
>>>>>> Yibo Zhao <yiboz@codeaurora.org> writes:
>>>>>>=20
>>>>>>> On 2019-09-19 18:37, Toke H=C3=B8iland-J=C3=B8rgensen wrote:
>>>>>>>> Yibo Zhao <yiboz@codeaurora.org> writes:
>>>>>>>>=20
>>>>>>>>> On 2019-09-18 19:23, Toke H=C3=B8iland-J=C3=B8rgensen wrote:
>>>>>>>>>> Yibo Zhao <yiboz@codeaurora.org> writes:
>>>>>>>>>>=20
>>>>>>>>>>> On 2019-09-18 05:10, Toke H=C3=B8iland-J=C3=B8rgensen wrote:
>>>>>>>>>>>> Yibo Zhao <yiboz@codeaurora.org> writes:
>>>>>>>>>>>>=20
>>>>>>>>>>>>> In a loop txqs dequeue scenario, if the first txq in the
>>>>>>>>>>>>> rbtree
>>>>>>>>>>>>> gets
>>>>>>>>>>>>> removed from rbtree immediately in the=20
>>>>>>>>>>>>> ieee80211_return_txq(),
>>>>>>>>>>>>> the
>>>>>>>>>>>>> loop will break soon in the ieee80211_next_txq() due to
>>>>>>>>>>>>> schedule_pos
>>>>>>>>>>>>> not leading to the second txq in the rbtree. Thus, defering
>>>>>>>>>>>>> the
>>>>>>>>>>>>> removal right before the end of this schedule round.
>>>>>>>>>>>>>=20
>>>>>>>>>>>>> Co-developed-by: Yibo Zhao <yiboz@codeaurora.org>
>>>>>>>>>>>>> Signed-off-by: Yibo Zhao <yiboz@codeaurora.org>
>>>>>>>>>>>>> Signed-off-by: Toke H=C3=B8iland-J=C3=B8rgensen <toke@toke.dk=
>
>>>>>>>>>>>>=20
>>>>>>>>>>>> I didn't write this patch, so please don't use my sign-off.
>>>>>>>>>>>> I'll
>>>>>>>>>>>> add
>>>>>>>>>>>> ack or review tags as appropriate in reply; but a few=20
>>>>>>>>>>>> comments
>>>>>>>>>>>> first:
>>>>>>>>>>>>=20
>>>>>>>>>>>>> ---
>>>>>>>>>>>>>  include/net/mac80211.h     | 16 ++++++++++--
>>>>>>>>>>>>>  net/mac80211/ieee80211_i.h |  3 +++
>>>>>>>>>>>>>  net/mac80211/main.c        |  6 +++++
>>>>>>>>>>>>>  net/mac80211/tx.c          | 63
>>>>>>>>>>>>> +++++++++++++++++++++++++++++++++++++++++++---
>>>>>>>>>>>>>  4 files changed, 83 insertions(+), 5 deletions(-)
>>>>>>>>>>>>>=20
>>>>>>>>>>>>> diff --git a/include/net/mac80211.h b/include/net/mac80211.h
>>>>>>>>>>>>> index ac2ed8e..ba5a345 100644
>>>>>>>>>>>>> --- a/include/net/mac80211.h
>>>>>>>>>>>>> +++ b/include/net/mac80211.h
>>>>>>>>>>>>> @@ -925,6 +925,8 @@ struct ieee80211_tx_rate {
>>>>>>>>>>>>>=20
>>>>>>>>>>>>>  #define IEEE80211_MAX_TX_RETRY=09=0931
>>>>>>>>>>>>>=20
>>>>>>>>>>>>> +#define IEEE80211_AIRTIME_TXQ_RM_CHK_INTV_IN_MS 100
>>>>>>>>>>>>> +
>>>>>>>>>>>>>  static inline void ieee80211_rate_set_vht(struct
>>>>>>>>>>>>> ieee80211_tx_rate
>>>>>>>>>>>>> *rate,
>>>>>>>>>>>>>  =09=09=09=09=09  u8 mcs, u8 nss)
>>>>>>>>>>>>>  {
>>>>>>>>>>>>> @@ -6232,7 +6234,8 @@ struct sk_buff
>>>>>>>>>>>>> *ieee80211_tx_dequeue(struct
>>>>>>>>>>>>> ieee80211_hw *hw,
>>>>>>>>>>>>>   * @ac: AC number to return packets from.
>>>>>>>>>>>>>   *
>>>>>>>>>>>>>   * Should only be called between calls to
>>>>>>>>>>>>> ieee80211_txq_schedule_start()
>>>>>>>>>>>>> - * and ieee80211_txq_schedule_end().
>>>>>>>>>>>>> + * and ieee80211_txq_schedule_end(). If the txq is empty,=20
>>>>>>>>>>>>> it
>>>>>>>>>>>>> will
>>>>>>>>>>>>> be
>>>>>>>>>>>>> added
>>>>>>>>>>>>> + * to a remove list and get removed later.
>>>>>>>>>>>>>   * Returns the next txq if successful, %NULL if no queue is
>>>>>>>>>>>>> eligible.
>>>>>>>>>>>>> If a txq
>>>>>>>>>>>>>   * is returned, it should be returned with
>>>>>>>>>>>>> ieee80211_return_txq()
>>>>>>>>>>>>> after the
>>>>>>>>>>>>>   * driver has finished scheduling it.
>>>>>>>>>>>>> @@ -6268,7 +6271,8 @@ void=20
>>>>>>>>>>>>> ieee80211_txq_schedule_start(struct
>>>>>>>>>>>>> ieee80211_hw *hw, u8 ac)
>>>>>>>>>>>>>   * @hw: pointer as obtained from ieee80211_alloc_hw()
>>>>>>>>>>>>>   * @ac: AC number to acquire locks for
>>>>>>>>>>>>>   *
>>>>>>>>>>>>> - * Release locks previously acquired by
>>>>>>>>>>>>> ieee80211_txq_schedule_end().
>>>>>>>>>>>>> + * Release locks previously acquired by
>>>>>>>>>>>>> ieee80211_txq_schedule_end().
>>>>>>>>>>>>> Check
>>>>>>>>>>>>> + * and remove the empty txq from rb-tree.
>>>>>>>>>>>>>   */
>>>>>>>>>>>>>  void ieee80211_txq_schedule_end(struct ieee80211_hw *hw, u8
>>>>>>>>>>>>> ac)
>>>>>>>>>>>>>  =09__releases(txq_lock);
>>>>>>>>>>>>> @@ -6287,6 +6291,14 @@ void ieee80211_schedule_txq(struct
>>>>>>>>>>>>> ieee80211_hw
>>>>>>>>>>>>> *hw, struct ieee80211_txq *txq)
>>>>>>>>>>>>>  =09__acquires(txq_lock) __releases(txq_lock);
>>>>>>>>>>>>>=20
>>>>>>>>>>>>>  /**
>>>>>>>>>>>>> + * ieee80211_txqs_check - Check txqs waiting for removal
>>>>>>>>>>>>> + *
>>>>>>>>>>>>> + * @tmr: pointer as obtained from local
>>>>>>>>>>>>> + *
>>>>>>>>>>>>> + */
>>>>>>>>>>>>> +void ieee80211_txqs_check(struct timer_list *tmr);
>>>>>>>>>>>>> +
>>>>>>>>>>>>> +/**
>>>>>>>>>>>>>   * ieee80211_txq_may_transmit - check whether TXQ is=20
>>>>>>>>>>>>> allowed
>>>>>>>>>>>>> to
>>>>>>>>>>>>> transmit
>>>>>>>>>>>>>   *
>>>>>>>>>>>>>   * This function is used to check whether given txq is
>>>>>>>>>>>>> allowed
>>>>>>>>>>>>> to
>>>>>>>>>>>>> transmit by
>>>>>>>>>>>>> diff --git a/net/mac80211/ieee80211_i.h
>>>>>>>>>>>>> b/net/mac80211/ieee80211_i.h
>>>>>>>>>>>>> index a4556f9..49aa143e 100644
>>>>>>>>>>>>> --- a/net/mac80211/ieee80211_i.h
>>>>>>>>>>>>> +++ b/net/mac80211/ieee80211_i.h
>>>>>>>>>>>>> @@ -847,6 +847,7 @@ struct txq_info {
>>>>>>>>>>>>>  =09struct codel_stats cstats;
>>>>>>>>>>>>>  =09struct sk_buff_head frags;
>>>>>>>>>>>>>  =09struct rb_node schedule_order;
>>>>>>>>>>>>> +=09struct list_head candidate;
>>>>>>>>>>>>>  =09unsigned long flags;
>>>>>>>>>>>>>=20
>>>>>>>>>>>>>  =09/* keep last! */
>>>>>>>>>>>>> @@ -1145,6 +1146,8 @@ struct ieee80211_local {
>>>>>>>>>>>>>  =09u64 airtime_v_t[IEEE80211_NUM_ACS];
>>>>>>>>>>>>>  =09u64 airtime_weight_sum[IEEE80211_NUM_ACS];
>>>>>>>>>>>>>=20
>>>>>>>>>>>>> +=09struct list_head remove_list[IEEE80211_NUM_ACS];
>>>>>>>>>>>>> +=09struct timer_list remove_timer;
>>>>>>>>>>>>>  =09u16 airtime_flags;
>>>>>>>>>>>>>=20
>>>>>>>>>>>>>  =09const struct ieee80211_ops *ops;
>>>>>>>>>>>>> diff --git a/net/mac80211/main.c b/net/mac80211/main.c
>>>>>>>>>>>>> index e9ffa8e..78fe24a 100644
>>>>>>>>>>>>> --- a/net/mac80211/main.c
>>>>>>>>>>>>> +++ b/net/mac80211/main.c
>>>>>>>>>>>>> @@ -667,10 +667,15 @@ struct ieee80211_hw
>>>>>>>>>>>>> *ieee80211_alloc_hw_nm(size_t priv_data_len,
>>>>>>>>>>>>>=20
>>>>>>>>>>>>>  =09for (i =3D 0; i < IEEE80211_NUM_ACS; i++) {
>>>>>>>>>>>>>  =09=09local->active_txqs[i] =3D RB_ROOT_CACHED;
>>>>>>>>>>>>> +=09=09INIT_LIST_HEAD(&local->remove_list[i]);
>>>>>>>>>>>>>  =09=09spin_lock_init(&local->active_txq_lock[i]);
>>>>>>>>>>>>>  =09}
>>>>>>>>>>>>>  =09local->airtime_flags =3D AIRTIME_USE_TX | AIRTIME_USE_RX;
>>>>>>>>>>>>>=20
>>>>>>>>>>>>> +=09timer_setup(&local->remove_timer, ieee80211_txqs_check,=
=20
>>>>>>>>>>>>> 0);
>>>>>>>>>>>>> +=09mod_timer(&local->remove_timer,
>>>>>>>>>>>>> +=09=09  jiffies +
>>>>>>>>>>>>> msecs_to_jiffies(IEEE80211_AIRTIME_TXQ_RM_CHK_INTV_IN_MS));
>>>>>>>>>>>>> +
>>>>>>>>>>>>>  =09INIT_LIST_HEAD(&local->chanctx_list);
>>>>>>>>>>>>>  =09mutex_init(&local->chanctx_mtx);
>>>>>>>>>>>>>=20
>>>>>>>>>>>>> @@ -1305,6 +1310,7 @@ void ieee80211_unregister_hw(struct
>>>>>>>>>>>>> ieee80211_hw
>>>>>>>>>>>>> *hw)
>>>>>>>>>>>>>  =09tasklet_kill(&local->tx_pending_tasklet);
>>>>>>>>>>>>>  =09tasklet_kill(&local->tasklet);
>>>>>>>>>>>>>=20
>>>>>>>>>>>>> +=09del_timer_sync(&local->remove_timer);
>>>>>>>>>>>>>  #ifdef CONFIG_INET
>>>>>>>>>>>>>  =09unregister_inetaddr_notifier(&local->ifa_notifier);
>>>>>>>>>>>>>  #endif
>>>>>>>>>>>>> diff --git a/net/mac80211/tx.c b/net/mac80211/tx.c
>>>>>>>>>>>>> index d00baaa..42ca010 100644
>>>>>>>>>>>>> --- a/net/mac80211/tx.c
>>>>>>>>>>>>> +++ b/net/mac80211/tx.c
>>>>>>>>>>>>> @@ -1450,6 +1450,7 @@ void ieee80211_txq_init(struct
>>>>>>>>>>>>> ieee80211_sub_if_data *sdata,
>>>>>>>>>>>>>  =09codel_stats_init(&txqi->cstats);
>>>>>>>>>>>>>  =09__skb_queue_head_init(&txqi->frags);
>>>>>>>>>>>>>  =09RB_CLEAR_NODE(&txqi->schedule_order);
>>>>>>>>>>>>> +=09INIT_LIST_HEAD(&txqi->candidate);
>>>>>>>>>>>>>=20
>>>>>>>>>>>>>  =09txqi->txq.vif =3D &sdata->vif;
>>>>>>>>>>>>>=20
>>>>>>>>>>>>> @@ -3724,6 +3725,9 @@ void ieee80211_schedule_txq(struct
>>>>>>>>>>>>> ieee80211_hw
>>>>>>>>>>>>> *hw,
>>>>>>>>>>>>>=20
>>>>>>>>>>>>>  =09spin_lock_bh(&local->active_txq_lock[ac]);
>>>>>>>>>>>>>=20
>>>>>>>>>>>>> +=09if (!list_empty(&txqi->candidate))
>>>>>>>>>>>>> +=09=09list_del_init(&txqi->candidate);
>>>>>>>>>>>>> +
>>>>>>>>>>>>>  =09if (!RB_EMPTY_NODE(&txqi->schedule_order))
>>>>>>>>>>>>>  =09=09goto out;
>>>>>>>>>>>>>=20
>>>>>>>>>>>>> @@ -3783,6 +3787,20 @@ static void
>>>>>>>>>>>>> __ieee80211_unschedule_txq(struct
>>>>>>>>>>>>> ieee80211_hw *hw,
>>>>>>>>>>>>>  =09RB_CLEAR_NODE(&txqi->schedule_order);
>>>>>>>>>>>>>  }
>>>>>>>>>>>>>=20
>>>>>>>>>>>>> +void ieee80211_remove_txq(struct ieee80211_hw *hw,
>>>>>>>>>>>>> +=09=09=09  struct ieee80211_txq *txq)
>>>>>>>>>>>>> +{
>>>>>>>>>>>>> +=09struct ieee80211_local *local =3D hw_to_local(hw);
>>>>>>>>>>>>> +=09struct txq_info *txqi =3D to_txq_info(txq);
>>>>>>>>>>>>> +
>>>>>>>>>>>>> +=09lockdep_assert_held(&local->active_txq_lock[txq->ac]);
>>>>>>>>>>>>> +
>>>>>>>>>>>>> +=09if (!RB_EMPTY_NODE(&txqi->schedule_order)) {
>>>>>>>>>>>>> +=09=09__ieee80211_unschedule_txq(hw, txq);
>>>>>>>>>>>>> +=09=09list_del_init(&txqi->candidate);
>>>>>>>>>>>>> +=09}
>>>>>>>>>>>>> +}
>>>>>>>>>>>>> +
>>>>>>>>>>>>>  void ieee80211_unschedule_txq(struct ieee80211_hw *hw,
>>>>>>>>>>>>>  =09=09=09      struct ieee80211_txq *txq)
>>>>>>>>>>>>>  =09__acquires(txq_lock) __releases(txq_lock)
>>>>>>>>>>>>> @@ -3790,7 +3808,7 @@ void ieee80211_unschedule_txq(struct
>>>>>>>>>>>>> ieee80211_hw *hw,
>>>>>>>>>>>>>  =09struct ieee80211_local *local =3D hw_to_local(hw);
>>>>>>>>>>>>>=20
>>>>>>>>>>>>>  =09spin_lock_bh(&local->active_txq_lock[txq->ac]);
>>>>>>>>>>>>> -=09__ieee80211_unschedule_txq(hw, txq);
>>>>>>>>>>>>> +=09ieee80211_remove_txq(hw, txq);
>>>>>>>>>>>>>  =09spin_unlock_bh(&local->active_txq_lock[txq->ac]);
>>>>>>>>>>>>>  }
>>>>>>>>>>>>>=20
>>>>>>>>>>>>> @@ -3803,11 +3821,48 @@ void ieee80211_return_txq(struct
>>>>>>>>>>>>> ieee80211_hw
>>>>>>>>>>>>> *hw,
>>>>>>>>>>>>>  =09lockdep_assert_held(&local->active_txq_lock[txq->ac]);
>>>>>>>>>>>>>=20
>>>>>>>>>>>>>  =09if (!RB_EMPTY_NODE(&txqi->schedule_order) &&
>>>>>>>>>>>>> -=09    (skb_queue_empty(&txqi->frags) &&
>>>>>>>>>>>>> !txqi->tin.backlog_packets))
>>>>>>>>>>>>> -=09=09__ieee80211_unschedule_txq(hw, txq);
>>>>>>>>>>>>> +=09=09!txq_has_queue(&txqi->txq) &&
>>>>>>>>>>>>> +=09=09list_empty(&txqi->candidate))
>>>>>>>>>>>>> +=09=09list_add_tail(&txqi->candidate,
>>>>>>>>>>>>> &local->remove_list[txq->ac]);
>>>>>>>>>>>>> +
>>>>>>>>>>>>>  }
>>>>>>>>>>>>>  EXPORT_SYMBOL(ieee80211_return_txq);
>>>>>>>>>>>>>=20
>>>>>>>>>>>>> +void __ieee80211_check_txqs(struct ieee80211_local *local,
>>>>>>>>>>>>> int
>>>>>>>>>>>>> ac)
>>>>>>>>>>>>> +{
>>>>>>>>>>>>> +=09struct txq_info *iter, *tmp;
>>>>>>>>>>>>> +=09struct sta_info *sta;
>>>>>>>>>>>>> +
>>>>>>>>>>>>> +=09lockdep_assert_held(&local->active_txq_lock[ac]);
>>>>>>>>>>>>> +
>>>>>>>>>>>>> +=09list_for_each_entry_safe(iter, tmp,=20
>>>>>>>>>>>>> &local->remove_list[ac],
>>>>>>>>>>>>> +=09=09=09=09 candidate) {
>>>>>>>>>>>>> +=09=09sta =3D container_of(iter->txq.sta, struct sta_info, s=
ta);
>>>>>>>>>>>>> +
>>>>>>>>>>>>> +=09=09if (txq_has_queue(&iter->txq))
>>>>>>>>>>>>> +=09=09=09list_del_init(&iter->candidate);
>>>>>>>>>>>>> +=09=09else
>>>>>>>>>>>>> +=09=09=09ieee80211_remove_txq(&local->hw, &iter->txq);
>>>>>>>>>>>>> +=09}
>>>>>>>>>>>>> +}
>>>>>>>>>>>>> +
>>>>>>>>>>>>> +void ieee80211_txqs_check(struct timer_list *t)
>>>>>>>>>>>>> +{
>>>>>>>>>>>>> +=09struct ieee80211_local *local =3D from_timer(local, t,
>>>>>>>>>>>>> remove_timer);
>>>>>>>>>>>>> +=09struct txq_info *iter, *tmp;
>>>>>>>>>>>>> +=09struct sta_info *sta;
>>>>>>>>>>>>> +=09int ac;
>>>>>>>>>>>>> +
>>>>>>>>>>>>> +=09for (ac =3D 0; ac < IEEE80211_NUM_ACS; ac++) {
>>>>>>>>>>>>> +=09=09spin_lock_bh(&local->active_txq_lock[ac]);
>>>>>>>>>>>>> +=09=09__ieee80211_check_txqs(local, ac);
>>>>>>>>>>>>> +=09=09spin_unlock_bh(&local->active_txq_lock[ac]);
>>>>>>>>>>>>> +=09}
>>>>>>>>>>>>> +
>>>>>>>>>>>>> +=09mod_timer(&local->remove_timer,
>>>>>>>>>>>>> +=09=09  jiffies +
>>>>>>>>>>>>> msecs_to_jiffies(IEEE80211_AIRTIME_TXQ_RM_CHK_INTV_IN_MS));
>>>>>>>>>>>>> +}
>>>>>>>>>>>>=20
>>>>>>>>>>>> I'll ask the same as I did last time (where you told me to=20
>>>>>>>>>>>> hold
>>>>>>>>>>>> off
>>>>>>>>>>>> until this round):
>>>>>>>>>>>>=20
>>>>>>>>>>>> Why do you need the timer and the periodic check? If TXQs are
>>>>>>>>>>>> added
>>>>>>>>>>>> to
>>>>>>>>>>>> the remove list during the scheduling run, and
>>>>>>>>>>>> __ieee80211_check_txqs()
>>>>>>>>>>>> is run from schedule_end(), isn't that sufficient to clear=20
>>>>>>>>>>>> the
>>>>>>>>>>>> list?
>>>>>>>>>>> Is it possible that a txq is not added to the remove list but
>>>>>>>>>>> then
>>>>>>>>>>> packets in it are dropped by fq_codel algo? Like the station
>>>>>>>>>>> disconnects
>>>>>>>>>>> without any notification.
>>>>>>>>>>=20
>>>>>>>>>> Well as long as all the other cleanup paths call directly into
>>>>>>>>>> __unschedule_txq(), that should remove stations from the
>>>>>>>>>> scheduler
>>>>>>>>>> when
>>>>>>>>>> they disconnect etc.
>>>>>>>>> Yes, the disconnect scenario is a bad example. My concern is,=20
>>>>>>>>> say,
>>>>>>>>> we
>>>>>>>>> have 10 stations and only one of them is assigned a very small
>>>>>>>>> weight
>>>>>>>>> compared with that of others. Suppose, after its chance of Tx,=20
>>>>>>>>> it
>>>>>>>>> is
>>>>>>>>> most likely to be placed in the rightmost(still has some packets
>>>>>>>>> in
>>>>>>>>> the
>>>>>>>>> txq) and no more incoming data for it. The remaining packets in
>>>>>>>>> txq
>>>>>>>>> will
>>>>>>>>> be dropped due to timeout algo in codel(correct me if I am=20
>>>>>>>>> wrong)
>>>>>>>>> but
>>>>>>>>> this empty txq will stay on the rbtree until other txqs get
>>>>>>>>> drained
>>>>>>>>> or
>>>>>>>>> global vt catch up with its vt. The staying time could be long=20
>>>>>>>>> if
>>>>>>>>> weight
>>>>>>>>> is extremely small. Then do we need timer to check or any other
>>>>>>>>> better
>>>>>>>>> solution?
>>>>>>>>=20
>>>>>>>> Ah, I see what you mean. No, I don't think this will be a=20
>>>>>>>> problem;
>>>>>>>> the
>>>>>>>> scenario you're describing would play out like this:
>>>>>>>>=20
>>>>>>>> 1. Station ends transmitting, still has a single packet queued,
>>>>>>>> gets
>>>>>>>>    moved to the end of the rbtree (and stays there for a while).
>>>>>>>>=20
>>>>>>>> 2. When we finally get to the point where this station gets=20
>>>>>>>> another
>>>>>>>>    chance to transmit, the CoDel drop timer triggers and the last
>>>>>>>> packet
>>>>>>>>    is dropped[0]. This means that the queue will just be empty
>>>>>>>>    (and ieee80211_tx_dequeue() will return NULL).
>>>>>>>>=20
>>>>>>>> 3. Because the queue is empty, ieee80211_return_txq() will not=20
>>>>>>>> put
>>>>>>>> it
>>>>>>>>    back on the rbtree.
>>>>>>>>=20
>>>>>>>> Crucially, in 2. the CoDel algorithm doesn't kick in until the
>>>>>>>> point
>>>>>>>> of
>>>>>>>> packet dequeue. But even if an empty queue stays on the rbtree=20
>>>>>>>> for
>>>>>>>> a
>>>>>>>> while, there is no harm in that: eventually it will get its turn,
>>>>>>>> it
>>>>>>>> will turn out to be empty, and just be skipped over.
>>>>>>> Then that will be fine. Thanks for the explanation of the dropping
>>>>>>> part
>>>>>>> in CoDel algorithm.
>>>>>>=20
>>>>>> Yup, think so. And you're welcome :)
>>>>>>=20
>>>>>>>> The issue we need to be concerned about is the opposite: If we=20
>>>>>>>> have
>>>>>>>> a
>>>>>>>> queue that *does* have packets queued, but which is *not*=20
>>>>>>>> scheduled
>>>>>>>> for
>>>>>>>> transmission, that will stall TX.
>>>>>>> Is it by design since its vt is more than global vt, right? The
>>>>>>> lattency
>>>>>>> may somehow get impacted though.
>>>>>>=20
>>>>>> Well, it should still stay on the rbtree as long as it has packets
>>>>>> queued. We don't have a check anywhere that reschedules TXQs whose
>>>>>> v_t
>>>>>> drops below global v_t...
>>>>>>=20
>>>>>>>> [0] CoDel in most cases only drops a single packet at a time, so=
=20
>>>>>>>> it
>>>>>>>> will
>>>>>>>> not clear out an entire queue with multiple packets in one go.=20
>>>>>>>> But
>>>>>>>> you
>>>>>>>> are right that it could conceivably drop the last packet in a
>>>>>>>> queue.
>>>>>>>>=20
>>>>>>>>>> We only need to defer removal inside a single "scheduling=20
>>>>>>>>>> round"
>>>>>>>>>> (i.e.,
>>>>>>>>>> between a pair of ieee80211_txq_schedule_start/end. So if we=20
>>>>>>>>>> just
>>>>>>>>>> walk
>>>>>>>>>> the remove list in schedule_end() we should be enough, no?
>>>>>>>>>>=20
>>>>>>>>>> Hmm, or maybe a simpler way to fix the original issue is just=20
>>>>>>>>>> to
>>>>>>>>>> have
>>>>>>>>>> unschedule_txq() update the schedule_pos() pointer?
>>>>>>>>>>=20
>>>>>>>>>> I.e., unschedule_txq checks if the txq being removed is=20
>>>>>>>>>> currently
>>>>>>>>>> being
>>>>>>>>>> pointed to by schedule_pos[ac], and if it is, it updates
>>>>>>>>>> schedule_pos
>>>>>>>>>> to
>>>>>>>>>> be the rb_next of the current value?
>>>>>>>>> Actually, if schedule_pos is updated to rb_next of the current
>>>>>>>>> value,
>>>>>>>>> then in the next_txq() where we are going to use rb_next again=20
>>>>>>>>> and
>>>>>>>>> finally pick the next node of the node we really want. Is it=20
>>>>>>>>> fine
>>>>>>>>> to
>>>>>>>>> update schedule_pos to NULL?
>>>>>>>>=20
>>>>>>>> Hmm, yeah, good point.
>>>>>>>>=20
>>>>>>>> If we do end up setting schedule_pos to NULL in the middle of a
>>>>>>>> scheduling round, that will make next_txq() "start over", and do
>>>>>>>> another
>>>>>>>> loop through the whole thing. I guess we may be able hit a case
>>>>>>>> where
>>>>>>>> things can oscillate back and forth between addition and removal
>>>>>>>> resulting in an infinite loop? Not sure, but at least I can't=20
>>>>>>>> seem
>>>>>>>> to
>>>>>>>> convince myself that this can't happen.
>>>>>>>=20
>>>>>>> As the loop of next_txq under lock protection as below,
>>>>>>>=20
>>>>>>> txq_schedule_start();
>>>>>>> while(txq=3Dnext_txq()){
>>>>>>> ...
>>>>>>> return_txq(txq);
>>>>>>> }
>>>>>>> txq_schedule_end();
>>>>>>>=20
>>>>>>> I do not see any chance of addition, no?
>>>>>>=20
>>>>>> As you noted in your other email, Felix reduced the locking. And
>>>>>> yeah,
>>>>>> we need to rebase this series to also incorporate that. I figure I
>>>>>> can
>>>>>> send an updated version of the first patch in the series once we've
>>>>>> worked out the remaining issues with your follow-up patches.
>>>>>>=20
>>>>> Oh, I was thinking we were discussing without locking reduced. Yes,=
=20
>>>>> I
>>>>> also agree there might be a case causing infinite loop. With locking
>>>>> reduced, the tree can be adjusted between next_txq() and=20
>>>>> return_txq()
>>>>> in
>>>>> the loop situation. For further discussion, let 's consider,
>>>>> 1) the tree starts like:
>>>>>         A->B->C->D->E
>>>>> 2) then next_txq() returns A for dequeuing
>>>>> 3) driver dequeues A and draines A without any active txq locked
>>>>> meaning
>>>>> the tree could be changed upon Tx compeletion.
>>>>> 4) then in return_txq(), the tree could be,
>>>>>         i   A->B->C->D->E =EF=BC=88A is empty, and maybe soon be adde=
d back
>>>>> before the loop end=EF=BC=89
>>>>>         ii  B->C->A->D->E =EF=BC=88A is empty, and maybe soon be adde=
d back
>>>>> before the loop end=EF=BC=89
>>>>>         iii B->C->D->E->A =EF=BC=88A is empty, and maybe soon be adde=
d back
>>>>> before the loop end)
>>>>>=20
>>>>> with this change:
>>>>>   local->schedule_pos[ac] =3D rb_next(node) ?: rb_prev(node);
>>>>>=20
>>>>> for case i, local->schedule_pos[ac] is rb_next(A) which is B, and in
>>>>> next_txq(), rb_next(B) is what we returns which actually is C and B=
=20
>>>>> is
>>>>> skipped, no?
>>>>>=20
>>>>> Similiar for case ii, we skip B, C, D.
>>>>=20
>>>> Yup, I think you're right. But if we can fix this by making
>>>> ieee80211_resort_txq() aware of the schedule_pos as well, no? I.e.,=20
>>>> if
>>>> resort_txq() acts on the txq that's currently in schedule_pos, it=20
>>>> will
>>>> update schedule pos with the same rb_next(node) ?: rb_prev(node);
>>>> (optionally after checking that the position of the node is actually
>>>> going to change).
>>> Sorry, please igore last email sent by mistake.
>>>=20
>>> I don't think it makes any difference with that in unschedule_txq().=20
>>> For
>>> case i, it finally picks C as well in next_txq(). For next_txq(),
>>> schedule_pos means previous candidate node whereas with your change,=20
>>> it
>>> looks like schedule_pos is current candidate node instead.
>>=20
>> Hmm, that was not actually what I was thinking, but yeah I think you're
>> right that it would be easier to just change it so schedule_pos is
>> pointing to the next and not the current txq we want to schedule.
> So do you mean we can change next_txq like this,
>
>   struct ieee80211_txq *ieee80211_next_txq(struct ieee80211_hw *hw, u8=20
> ac)
>   {
>   =09struct ieee80211_local *local =3D hw_to_local(hw);
> =09struct rb_node *node =3D local->schedule_pos[ac];
>   =09struct txq_info *txqi =3D NULL;
> =09bool first =3D false;
>
>   =09lockdep_assert_held(&local->active_txq_lock[ac]);
>
> =09if (!node) {
> =09        node =3D rb_first_cached(&local->active_txqs[ac]);
> =09=09first =3D true;
> -=09} else
> -=09=09node =3D rb_next(node);
> +       }
> +
> =09if (!node)
>   =09=09return NULL;

Ah, no, now I remember why this didn't work and I went with the other
approach: If you make this change, you also have to have this at the
end:

local->schedule_pos[ac] =3D rb_next(node);


But this means we can no longer distinguish between having gone through
the whole thing (so rb_next() returns NULL), or starting out with
nothing.

So, instead we need to keep next_txq() the way it is, and just add

local->schedule_pos[ac] =3D rb_prev(node);

whenever we remove a node (both in return_txq() and resort_txq()).

>>=20
>> We'd still need a check in resort_txq() then, but it would make it safe
>> to unschedule in return_txq()...
> Yes, agree with that.
>
>
>>=20
>>>>> Also I am wondering if there will be some SMP issues relating with
>>>>> local->schedule_pos[ac].
>>>>=20
>>>> Not sure what you mean by this?
>>> My bad. Please ignore this.
>>>=20
>>>=20
>>>>=20
>>>>>>> In ath10k, we will usually push packets of first txq as many as we
>>>>>>> can
>>>>>>> until it is drained and then move to the next one. So if a txq=20
>>>>>>> gets
>>>>>>> removed in the return_txq, it should always be the leftmost. And
>>>>>>> during this period, neither vt of any station or global vt can be
>>>>>>> updated due to lock protection.
>>>>>>>=20
>>>>>>>>=20
>>>>>>>> But in that case, we could fix it by just conditionally assigning
>>>>>>>> either
>>>>>>>> rb_next or rb_prev to the schedule_pos in unschedule_txq()? I.e.,
>>>>>>>> something like:
>>>>>>>>=20
>>>>>>>> local->schedule_pos[ac] =3D rb_next(node) ?: rb_prev(node);
>>>>>>> I am not sure I am getting your point. Still in next_txq,
>>>>>>> schedule_pos[ac] will lead us to the next node of the one we want.
>>>>>>=20
>>>>>> The logic in next_txq is different when schedule_pos[ac] is NULL,=20
>>>>>> vs
>>>>>> when rb_next(schedule_pos[ac]) is NULL. The former restarts a new
>>>>>> scheduling round, while the latter ends the current round.
>>>>>>=20
>>>>>> -Toke
>>>>>=20
>>>>> --
>>>>> Yibo
>>>=20
>>> --
>>> Yibo
>
> --=20
> Yibo