Re: [Make-wifi-fast] [RFC/RFT] mac80211: Switch to a virtual time-based airtime scheduler

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

Yibo Zhao <yiboz@codeaurora.org> writes:

> On 2019-04-10 18:40, Toke Høiland-Jørgensen wrote:
>> Yibo Zhao <yiboz@codeaurora.org> writes:
>> 
>>> On 2019-04-10 04:41, Toke Høiland-Jørgensen wrote:
>>>> Yibo Zhao <yiboz@codeaurora.org> writes:
>>>> 
>>>>> On 2019-04-04 16:31, Toke Høiland-Jørgensen wrote:
>>>>>> Yibo Zhao <yiboz@codeaurora.org> writes:
>>>>>> 
>>>>>>> On 2019-02-16 01:05, Toke Høiland-Jørgensen wrote:
>>>>>>>> This switches the airtime scheduler in mac80211 to use a virtual
>>>>>>>> time-based
>>>>>>>> scheduler instead of the round-robin scheduler used before. This
>>>>>>>> has
>>>>>>>> a
>>>>>>>> couple of advantages:
>>>>>>>> 
>>>>>>>> - No need to sync up the round-robin scheduler in 
>>>>>>>> firmware/hardware
>>>>>>>> with
>>>>>>>>   the round-robin airtime scheduler.
>>>>>>>> 
>>>>>>>> - If several stations are eligible for transmission we can 
>>>>>>>> schedule
>>>>>>>> both of
>>>>>>>>   them; no need to hard-block the scheduling rotation until the
>>>>>>>> head
>>>>>>>> of
>>>>>>>> the
>>>>>>>>   queue has used up its quantum.
>>>>>>>> 
>>>>>>>> - The check of whether a station is eligible for transmission
>>>>>>>> becomes
>>>>>>>>   simpler (in ieee80211_txq_may_transmit()).
>>>>>>>> 
>>>>>>>> The drawback is that scheduling becomes slightly more expensive, 
>>>>>>>> as
>>>>>>>> we
>>>>>>>> need
>>>>>>>> to maintain an rbtree of TXQs sorted by virtual time. This means
>>>>>>>> that
>>>>>>>> ieee80211_register_airtime() becomes O(logN) in the number of
>>>>>>>> currently
>>>>>>>> scheduled TXQs. However, hopefully this number rarely grows too 
>>>>>>>> big
>>>>>>>> (it's
>>>>>>>> only TXQs currently backlogged, not all associated stations), so 
>>>>>>>> it
>>>>>>>> shouldn't be too big of an issue.
>>>>>>>> 
>>>>>>>> @@ -1831,18 +1830,32 @@ void 
>>>>>>>> ieee80211_sta_register_airtime(struct
>>>>>>>> ieee80211_sta *pubsta, u8 tid,
>>>>>>>>  {
>>>>>>>>  	struct sta_info *sta = container_of(pubsta, struct sta_info,
>>>>>>>> sta);
>>>>>>>>  	struct ieee80211_local *local = sta->sdata->local;
>>>>>>>> +	struct ieee80211_txq *txq = sta->sta.txq[tid];
>>>>>>>>  	u8 ac = ieee80211_ac_from_tid(tid);
>>>>>>>> -	u32 airtime = 0;
>>>>>>>> +	u64 airtime = 0, weight_sum;
>>>>>>>> +
>>>>>>>> +	if (!txq)
>>>>>>>> +		return;
>>>>>>>> 
>>>>>>>>  	if (sta->local->airtime_flags & AIRTIME_USE_TX)
>>>>>>>>  		airtime += tx_airtime;
>>>>>>>>  	if (sta->local->airtime_flags & AIRTIME_USE_RX)
>>>>>>>>  		airtime += rx_airtime;
>>>>>>>> 
>>>>>>>> +	/* Weights scale so the unit weight is 256 */
>>>>>>>> +	airtime <<= 8;
>>>>>>>> +
>>>>>>>>  	spin_lock_bh(&local->active_txq_lock[ac]);
>>>>>>>> +
>>>>>>>>  	sta->airtime[ac].tx_airtime += tx_airtime;
>>>>>>>>  	sta->airtime[ac].rx_airtime += rx_airtime;
>>>>>>>> -	sta->airtime[ac].deficit -= airtime;
>>>>>>>> +
>>>>>>>> +	weight_sum = local->airtime_weight_sum[ac] ?:
>>>>>>>> sta->airtime_weight;
>>>>>>>> +
>>>>>>>> +	local->airtime_v_t[ac] += airtime / weight_sum;
>>>>>>> Hi Toke,
>>>>>>> 
>>>>>>> Please ignore the previous two broken emails regarding this new
>>>>>>> proposal
>>>>>>> from me.
>>>>>>> 
>>>>>>> It looks like local->airtime_v_t acts like a Tx criteria. Only the
>>>>>>> stations with less airtime than that are valid for Tx. That means
>>>>>>> there
>>>>>>> are situations, like 50 clients, that some of the stations can be
>>>>>>> used
>>>>>>> to Tx when putting next_txq in the loop. Am I right?
>>>>>> 
>>>>>> I'm not sure what you mean here. Are you referring to the case 
>>>>>> where
>>>>>> new
>>>>>> stations appear with a very low (zero) airtime_v_t? That is handled
>>>>>> when
>>>>>> the station is enqueued.
>>>>> Hi Toke,
>>>>> 
>>>>> Sorry for the confusion. I am not referring to the case that you
>>>>> mentioned though it can be solved by your subtle design, max(local 
>>>>> vt,
>>>>> sta vt). :-)
>>>>> 
>>>>> Actually, my concern is situation about putting next_txq in the 
>>>>> loop.
>>>>> Let me explain a little more and see below.
>>>>> 
>>>>>> @@ -3640,126 +3638,191 @@ EXPORT_SYMBOL(ieee80211_tx_dequeue);
>>>>>>  struct ieee80211_txq *ieee80211_next_txq(struct ieee80211_hw *hw, 
>>>>>> u8
>>>>>> ac)
>>>>>>  {
>>>>>>  	struct ieee80211_local *local = hw_to_local(hw);
>>>>>> +	struct rb_node *node = local->schedule_pos[ac];
>>>>>>  	struct txq_info *txqi = NULL;
>>>>>> +	bool first = false;
>>>>>> 
>>>>>>  	lockdep_assert_held(&local->active_txq_lock[ac]);
>>>>>> 
>>>>>> - begin:
>>>>>> -	txqi = list_first_entry_or_null(&local->active_txqs[ac],
>>>>>> -					struct txq_info,
>>>>>> -					schedule_order);
>>>>>> -	if (!txqi)
>>>>>> +	if (!node) {
>>>>>> +		node = rb_first_cached(&local->active_txqs[ac]);
>>>>>> +		first = true;
>>>>>> +	} else
>>>>>> +		node = rb_next(node);
>>>>> 
>>>>> Consider below piece of code from ath10k_mac_schedule_txq:
>>>>> 
>>>>>          ieee80211_txq_schedule_start(hw, ac);
>>>>>          while ((txq = ieee80211_next_txq(hw, ac))) {
>>>>>                  while (ath10k_mac_tx_can_push(hw, txq)) {
>>>>>                          ret = ath10k_mac_tx_push_txq(hw, txq);
>>>>>                          if (ret < 0)
>>>>>                                  break;
>>>>>                  }
>>>>>                  ieee80211_return_txq(hw, txq);
>>>>>                  ath10k_htt_tx_txq_update(hw, txq);
>>>>>                  if (ret == -EBUSY)
>>>>>                          break;
>>>>>          }
>>>>>          ieee80211_txq_schedule_end(hw, ac);
>>>>> 
>>>>> If my understanding is right, local->schedule_pos is used to record
>>>>> the
>>>>> last scheduled node and used for traversal rbtree for valid txq. 
>>>>> There
>>>>> is chance that an empty txq is feeded to return_txq and got removed
>>>>> from
>>>>> rbtree. The empty txq will always be the rb_first node. Then in the
>>>>> following next_txq, local->schedule_pos becomes meaningless since 
>>>>> its
>>>>> rb_next will return NULL and the loop break. Only rb_first get
>>>>> dequeued
>>>>> during this loop.
>>>>> 
>>>>> 	if (!node || RB_EMPTY_NODE(node)) {
>>>>> 		node = rb_first_cached(&local->active_txqs[ac]);
>>>>> 		first = true;
>>>>> 	} else
>>>>> 		node = rb_next(node);
>>>> 
>>>> Ah, I see what you mean. Yes, that would indeed be a problem - nice
>>>> catch! :)
>>>> 
>>>>> How about this? The nodes on the rbtree will be dequeued and removed
>>>>> from rbtree one by one until HW is busy. Please note local vt and 
>>>>> sta
>>>>> vt will not be updated since txq lock is held during this time.
>>>> 
>>>> Insertion and removal from the rbtree are relatively expensive, so 
>>>> I'd
>>>> rather not do that for every txq. I think a better way to solve this
>>>> is to just defer the actual removal from the tree until
>>>> ieee80211_txq_schedule_end()... Will fix that when I submit this 
>>>> again.
>>> 
>>> Do you mean we keep the empty txqs in the rbtree until loop finishes 
>>> and
>>> remove them in ieee80211_txq_schedule_end(may be put return_txq in 
>>> it)?
>>> If it is the case, I suppose a list is needed to store the empty txqs 
>>> so
>>> as to dequeue them in ieee80211_txq_schedule_end.
>> 
>> Yeah, return_txq() would just put "to be removed" TXQs on a list, and
>> schedule_end() would do the actual removal (after checking whether a 
>> new
>> packet showed up in the meantime).
>
> SGTM
>
>> 
>>> And one more thing,
>>> 
>>>> +               if (sta->airtime[ac].v_t > local->airtime_v_t[ac]) {
>>>> +                       if (first)
>>>> +                               local->airtime_v_t[ac] =
>>>> sta->airtime[ac].v_t;
>>>> +                       else
>>>> +                               return NULL;
>>> 
>>> As local->airtime_v_t will not be updated during loop, we don't need 
>>> to
>>> return NULL.
>> 
>> Yes we do; this is actually the break condition. I.e., stations whose
>> virtual time are higher than the global time (in local->airtime_v_t) 
>> are
>> not allowed to transmit. And since we are traversing them in order, 
>> when
>> we find the first such station, we are done and can break out of the
>> scheduling loop entirely (which is what we do by returning NULL). The
>> other branch in the inner if() is just for the case where no stations
>> are currently eligible to transmit according to this rule; here we 
>> don't
>> want to stall, so we advance the global timer so the first station
>> becomes eligible...
>
> Yes，the inner if() make sure first node always get scheduled no matter 
> its vt.
>
> To detail my concern, let's assume only two nodes in the tree and
> empty nodes will be in tree until schedule_end(). In the loop and in
> case hw is not busy, ath10k will drain every node next_txq returned
> before asking for another txq again. Then as we are traversing to next
> rb node, it is highly possible the second node is not allowed to
> transmit since the global time has not been updated yet as the active
> txq lock is held. At this time, only second node on the tree has data
> and hw is capable of sending more data. I don't think the second node
> is not valid for transmission in this situation.
>
> With more nodes in the tree in this situation, I think same thing
> happens that all nodes except the first node are not allowed to
> transmit since none of their vts are less than the global time which
> is not updated in time. The loop breaks when we are checking the
> second node.

Yeah, in many cases we will end up throttling all but the first (couple
of) node(s). This is by design; otherwise we can't ensure fairness. As
long as we are making forward progress that is fine, though...

-Toke