Re: [PATCH 2/4] mac80211: defer txqs removal from rbtree

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Yibo Zhao <yiboz@codeaurora.org> writes:

> On 2019-09-20 17:15, Toke Høiland-Jørgensen wrote:
>> Yibo Zhao <yiboz@codeaurora.org> writes:
>> 
>>> On 2019-09-19 18:37, Toke Høiland-Jørgensen wrote:
>>>> Yibo Zhao <yiboz@codeaurora.org> writes:
>>>> 
>>>>> On 2019-09-18 19:23, Toke Høiland-Jørgensen wrote:
>>>>>> Yibo Zhao <yiboz@codeaurora.org> writes:
>>>>>> 
>>>>>>> On 2019-09-18 05:10, Toke Høiland-Jørgensen wrote:
>>>>>>>> Yibo Zhao <yiboz@codeaurora.org> writes:
>>>>>>>> 
>>>>>>>>> In a loop txqs dequeue scenario, if the first txq in the rbtree
>>>>>>>>> gets
>>>>>>>>> removed from rbtree immediately in the 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.
>>>>>>>>> 
>>>>>>>>> Co-developed-by: Yibo Zhao <yiboz@codeaurora.org>
>>>>>>>>> Signed-off-by: Yibo Zhao <yiboz@codeaurora.org>
>>>>>>>>> Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>
>>>>>>>> 
>>>>>>>> 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 comments
>>>>>>>> first:
>>>>>>>> 
>>>>>>>>> ---
>>>>>>>>>  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(-)
>>>>>>>>> 
>>>>>>>>> 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 {
>>>>>>>>> 
>>>>>>>>>  #define IEEE80211_MAX_TX_RETRY		31
>>>>>>>>> 
>>>>>>>>> +#define IEEE80211_AIRTIME_TXQ_RM_CHK_INTV_IN_MS 100
>>>>>>>>> +
>>>>>>>>>  static inline void ieee80211_rate_set_vht(struct
>>>>>>>>> ieee80211_tx_rate
>>>>>>>>> *rate,
>>>>>>>>>  					  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, 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 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)
>>>>>>>>>  	__releases(txq_lock);
>>>>>>>>> @@ -6287,6 +6291,14 @@ void ieee80211_schedule_txq(struct
>>>>>>>>> ieee80211_hw
>>>>>>>>> *hw, struct ieee80211_txq *txq)
>>>>>>>>>  	__acquires(txq_lock) __releases(txq_lock);
>>>>>>>>> 
>>>>>>>>>  /**
>>>>>>>>> + * 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 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 {
>>>>>>>>>  	struct codel_stats cstats;
>>>>>>>>>  	struct sk_buff_head frags;
>>>>>>>>>  	struct rb_node schedule_order;
>>>>>>>>> +	struct list_head candidate;
>>>>>>>>>  	unsigned long flags;
>>>>>>>>> 
>>>>>>>>>  	/* keep last! */
>>>>>>>>> @@ -1145,6 +1146,8 @@ struct ieee80211_local {
>>>>>>>>>  	u64 airtime_v_t[IEEE80211_NUM_ACS];
>>>>>>>>>  	u64 airtime_weight_sum[IEEE80211_NUM_ACS];
>>>>>>>>> 
>>>>>>>>> +	struct list_head remove_list[IEEE80211_NUM_ACS];
>>>>>>>>> +	struct timer_list remove_timer;
>>>>>>>>>  	u16 airtime_flags;
>>>>>>>>> 
>>>>>>>>>  	const 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,
>>>>>>>>> 
>>>>>>>>>  	for (i = 0; i < IEEE80211_NUM_ACS; i++) {
>>>>>>>>>  		local->active_txqs[i] = RB_ROOT_CACHED;
>>>>>>>>> +		INIT_LIST_HEAD(&local->remove_list[i]);
>>>>>>>>>  		spin_lock_init(&local->active_txq_lock[i]);
>>>>>>>>>  	}
>>>>>>>>>  	local->airtime_flags = AIRTIME_USE_TX | AIRTIME_USE_RX;
>>>>>>>>> 
>>>>>>>>> +	timer_setup(&local->remove_timer, ieee80211_txqs_check, 0);
>>>>>>>>> +	mod_timer(&local->remove_timer,
>>>>>>>>> +		  jiffies +
>>>>>>>>> msecs_to_jiffies(IEEE80211_AIRTIME_TXQ_RM_CHK_INTV_IN_MS));
>>>>>>>>> +
>>>>>>>>>  	INIT_LIST_HEAD(&local->chanctx_list);
>>>>>>>>>  	mutex_init(&local->chanctx_mtx);
>>>>>>>>> 
>>>>>>>>> @@ -1305,6 +1310,7 @@ void ieee80211_unregister_hw(struct
>>>>>>>>> ieee80211_hw
>>>>>>>>> *hw)
>>>>>>>>>  	tasklet_kill(&local->tx_pending_tasklet);
>>>>>>>>>  	tasklet_kill(&local->tasklet);
>>>>>>>>> 
>>>>>>>>> +	del_timer_sync(&local->remove_timer);
>>>>>>>>>  #ifdef CONFIG_INET
>>>>>>>>>  	unregister_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,
>>>>>>>>>  	codel_stats_init(&txqi->cstats);
>>>>>>>>>  	__skb_queue_head_init(&txqi->frags);
>>>>>>>>>  	RB_CLEAR_NODE(&txqi->schedule_order);
>>>>>>>>> +	INIT_LIST_HEAD(&txqi->candidate);
>>>>>>>>> 
>>>>>>>>>  	txqi->txq.vif = &sdata->vif;
>>>>>>>>> 
>>>>>>>>> @@ -3724,6 +3725,9 @@ void ieee80211_schedule_txq(struct
>>>>>>>>> ieee80211_hw
>>>>>>>>> *hw,
>>>>>>>>> 
>>>>>>>>>  	spin_lock_bh(&local->active_txq_lock[ac]);
>>>>>>>>> 
>>>>>>>>> +	if (!list_empty(&txqi->candidate))
>>>>>>>>> +		list_del_init(&txqi->candidate);
>>>>>>>>> +
>>>>>>>>>  	if (!RB_EMPTY_NODE(&txqi->schedule_order))
>>>>>>>>>  		goto out;
>>>>>>>>> 
>>>>>>>>> @@ -3783,6 +3787,20 @@ static void
>>>>>>>>> __ieee80211_unschedule_txq(struct
>>>>>>>>> ieee80211_hw *hw,
>>>>>>>>>  	RB_CLEAR_NODE(&txqi->schedule_order);
>>>>>>>>>  }
>>>>>>>>> 
>>>>>>>>> +void ieee80211_remove_txq(struct ieee80211_hw *hw,
>>>>>>>>> +			  struct ieee80211_txq *txq)
>>>>>>>>> +{
>>>>>>>>> +	struct ieee80211_local *local = hw_to_local(hw);
>>>>>>>>> +	struct txq_info *txqi = to_txq_info(txq);
>>>>>>>>> +
>>>>>>>>> +	lockdep_assert_held(&local->active_txq_lock[txq->ac]);
>>>>>>>>> +
>>>>>>>>> +	if (!RB_EMPTY_NODE(&txqi->schedule_order)) {
>>>>>>>>> +		__ieee80211_unschedule_txq(hw, txq);
>>>>>>>>> +		list_del_init(&txqi->candidate);
>>>>>>>>> +	}
>>>>>>>>> +}
>>>>>>>>> +
>>>>>>>>>  void ieee80211_unschedule_txq(struct ieee80211_hw *hw,
>>>>>>>>>  			      struct ieee80211_txq *txq)
>>>>>>>>>  	__acquires(txq_lock) __releases(txq_lock)
>>>>>>>>> @@ -3790,7 +3808,7 @@ void ieee80211_unschedule_txq(struct
>>>>>>>>> ieee80211_hw *hw,
>>>>>>>>>  	struct ieee80211_local *local = hw_to_local(hw);
>>>>>>>>> 
>>>>>>>>>  	spin_lock_bh(&local->active_txq_lock[txq->ac]);
>>>>>>>>> -	__ieee80211_unschedule_txq(hw, txq);
>>>>>>>>> +	ieee80211_remove_txq(hw, txq);
>>>>>>>>>  	spin_unlock_bh(&local->active_txq_lock[txq->ac]);
>>>>>>>>>  }
>>>>>>>>> 
>>>>>>>>> @@ -3803,11 +3821,48 @@ void ieee80211_return_txq(struct
>>>>>>>>> ieee80211_hw
>>>>>>>>> *hw,
>>>>>>>>>  	lockdep_assert_held(&local->active_txq_lock[txq->ac]);
>>>>>>>>> 
>>>>>>>>>  	if (!RB_EMPTY_NODE(&txqi->schedule_order) &&
>>>>>>>>> -	    (skb_queue_empty(&txqi->frags) &&
>>>>>>>>> !txqi->tin.backlog_packets))
>>>>>>>>> -		__ieee80211_unschedule_txq(hw, txq);
>>>>>>>>> +		!txq_has_queue(&txqi->txq) &&
>>>>>>>>> +		list_empty(&txqi->candidate))
>>>>>>>>> +		list_add_tail(&txqi->candidate, 
>>>>>>>>> &local->remove_list[txq->ac]);
>>>>>>>>> +
>>>>>>>>>  }
>>>>>>>>>  EXPORT_SYMBOL(ieee80211_return_txq);
>>>>>>>>> 
>>>>>>>>> +void __ieee80211_check_txqs(struct ieee80211_local *local, int
>>>>>>>>> ac)
>>>>>>>>> +{
>>>>>>>>> +	struct txq_info *iter, *tmp;
>>>>>>>>> +	struct sta_info *sta;
>>>>>>>>> +
>>>>>>>>> +	lockdep_assert_held(&local->active_txq_lock[ac]);
>>>>>>>>> +
>>>>>>>>> +	list_for_each_entry_safe(iter, tmp, &local->remove_list[ac],
>>>>>>>>> +				 candidate) {
>>>>>>>>> +		sta = container_of(iter->txq.sta, struct sta_info, sta);
>>>>>>>>> +
>>>>>>>>> +		if (txq_has_queue(&iter->txq))
>>>>>>>>> +			list_del_init(&iter->candidate);
>>>>>>>>> +		else
>>>>>>>>> +			ieee80211_remove_txq(&local->hw, &iter->txq);
>>>>>>>>> +	}
>>>>>>>>> +}
>>>>>>>>> +
>>>>>>>>> +void ieee80211_txqs_check(struct timer_list *t)
>>>>>>>>> +{
>>>>>>>>> +	struct ieee80211_local *local = from_timer(local, t,
>>>>>>>>> remove_timer);
>>>>>>>>> +	struct txq_info *iter, *tmp;
>>>>>>>>> +	struct sta_info *sta;
>>>>>>>>> +	int ac;
>>>>>>>>> +
>>>>>>>>> +	for (ac = 0; ac < IEEE80211_NUM_ACS; ac++) {
>>>>>>>>> +		spin_lock_bh(&local->active_txq_lock[ac]);
>>>>>>>>> +		__ieee80211_check_txqs(local, ac);
>>>>>>>>> +		spin_unlock_bh(&local->active_txq_lock[ac]);
>>>>>>>>> +	}
>>>>>>>>> +
>>>>>>>>> +	mod_timer(&local->remove_timer,
>>>>>>>>> +		  jiffies +
>>>>>>>>> msecs_to_jiffies(IEEE80211_AIRTIME_TXQ_RM_CHK_INTV_IN_MS));
>>>>>>>>> +}
>>>>>>>> 
>>>>>>>> I'll ask the same as I did last time (where you told me to hold 
>>>>>>>> off
>>>>>>>> until this round):
>>>>>>>> 
>>>>>>>> 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 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.
>>>>>> 
>>>>>> 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, 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, 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 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 if
>>>>> weight
>>>>> is extremely small. Then do we need timer to check or any other 
>>>>> better
>>>>> solution?
>>>> 
>>>> Ah, I see what you mean. No, I don't think this will be a problem; 
>>>> the
>>>> scenario you're describing would play out like this:
>>>> 
>>>> 1. Station ends transmitting, still has a single packet queued, gets
>>>>    moved to the end of the rbtree (and stays there for a while).
>>>> 
>>>> 2. When we finally get to the point where this station gets 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).
>>>> 
>>>> 3. Because the queue is empty, ieee80211_return_txq() will not put it
>>>>    back on the rbtree.
>>>> 
>>>> 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 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.
>> 
>> Yup, think so. And you're welcome :)
>> 
>>>> The issue we need to be concerned about is the opposite: If we have a
>>>> queue that *does* have packets queued, but which is *not* 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.
>> 
>> 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...
>> 
>>>> [0] CoDel in most cases only drops a single packet at a time, so it
>>>> will
>>>> not clear out an entire queue with multiple packets in one go. But 
>>>> you
>>>> are right that it could conceivably drop the last packet in a queue.
>>>> 
>>>>>> We only need to defer removal inside a single "scheduling round"
>>>>>> (i.e.,
>>>>>> between a pair of ieee80211_txq_schedule_start/end. So if we just
>>>>>> walk
>>>>>> the remove list in schedule_end() we should be enough, no?
>>>>>> 
>>>>>> Hmm, or maybe a simpler way to fix the original issue is just to 
>>>>>> have
>>>>>> unschedule_txq() update the schedule_pos() pointer?
>>>>>> 
>>>>>> I.e., unschedule_txq checks if the txq being removed is 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 and
>>>>> finally pick the next node of the node we really want. Is it fine to
>>>>> update schedule_pos to NULL?
>>>> 
>>>> Hmm, yeah, good point.
>>>> 
>>>> 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 seem to
>>>> convince myself that this can't happen.
>>> 
>>> As the loop of next_txq under lock protection as below,
>>> 
>>> txq_schedule_start();
>>> while(txq=next_txq()){
>>> ...
>>> return_txq(txq);
>>> }
>>> txq_schedule_end();
>>> 
>>> I do not see any chance of addition, no?
>> 
>> 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.
>> 
> Oh, I was thinking we were discussing without locking reduced. Yes, I 
> also agree there might be a case causing infinite loop. With locking 
> reduced, the tree can be adjusted between next_txq() and 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 （A is empty, and maybe soon be added back 
> before the loop end）
>         ii  B->C->A->D->E （A is empty, and maybe soon be added back 
> before the loop end）
>         iii B->C->D->E->A （A is empty, and maybe soon be added back 
> before the loop end)
>
> with this change:
>   local->schedule_pos[ac] = rb_next(node) ?: rb_prev(node);
>
> 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 is 
> skipped, no?
>
> Similiar for case ii, we skip B, C, D.

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., if
resort_txq() acts on the txq that's currently in schedule_pos, it 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).

> Also I am wondering if there will be some SMP issues relating with 
> local->schedule_pos[ac].

Not sure what you mean by this?

>>> 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 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.
>>> 
>>>> 
>>>> 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:
>>>> 
>>>> local->schedule_pos[ac] = 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.
>> 
>> The logic in next_txq is different when schedule_pos[ac] is NULL, vs
>> when rb_next(schedule_pos[ac]) is NULL. The former restarts a new
>> scheduling round, while the latter ends the current round.
>> 
>> -Toke
>
> -- 
> Yibo