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

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

Yibo Zhao <yiboz@codeaurora.org> writes:

> On 2019-09-18 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.

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?

-Toke