On 2019-09-21 19:27, Toke Høiland-Jørgensen wrote:
Yibo Zhao <yiboz@xxxxxxxxxxxxxx> writes:On 2019-09-20 17:15, Toke Høiland-Jørgensen wrote:Yibo Zhao <yiboz@xxxxxxxxxxxxxx> writes:On 2019-09-19 18:37, Toke Høiland-Jørgensen wrote:Yibo Zhao <yiboz@xxxxxxxxxxxxxx> writes:On 2019-09-18 19:23, Toke Høiland-Jørgensen wrote:Yibo Zhao <yiboz@xxxxxxxxxxxxxx> writes:On 2019-09-18 05:10, Toke Høiland-Jørgensen wrote:Is it possible that a txq is not added to the remove list but thenYibo Zhao <yiboz@xxxxxxxxxxxxxx> writes:In a loop txqs dequeue scenario, if the first txq in the rbtreegets removed from rbtree immediately in the ieee80211_return_txq(), the loop will break soon in the ieee80211_next_txq() due to schedule_posnot leading to the second txq in the rbtree. Thus, defering theremoval right before the end of this schedule round. Co-developed-by: Yibo Zhao <yiboz@xxxxxxxxxxxxxx> Signed-off-by: Yibo Zhao <yiboz@xxxxxxxxxxxxxx> Signed-off-by: Toke Høiland-Jørgensen <toke@xxxxxxx>I didn't write this patch, so please don't use my sign-off. I'lladd 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 totransmit ** This function is used to check whether given txq is allowedto 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, intac) +{ + 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?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 schedulerwhen 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 weightcompared with that of others. Suppose, after its chance of Tx, it is most likely to be placed in the rightmost(still has some packets inthetxq) and no more incoming data for it. The remaining packets in txqwill be dropped due to timeout algo in codel(correct me if I am wrong) butthis empty txq will stay on the rbtree until other txqs get drainedor 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, getsmoved 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 itback on the rbtree.Crucially, in 2. the CoDel algorithm doesn't kick in until the pointofpacket 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, itwill 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 aqueue 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 packetsqueued. We don't have a check anywhere that reschedules TXQs whose v_tdrops 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 youare 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 andfinally pick the next node of the node we really want. Is it fine toupdate 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 anotherloop through the whole thing. I guess we may be able hit a case wherethings can oscillate back and forth between addition and removalresulting in an infinite loop? Not sure, but at least I can't seem toconvince 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 cansend 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 lockingreduced, the tree can be adjusted between next_txq() and return_txq() inthe 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 dequeuing3) driver dequeues A and draines A without any active txq locked meaningthe 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 canuntil 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
-- Yibo
