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

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

Yibo Zhao <yiboz@codeaurora.org> writes:

> On 2019-09-19 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.

> 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