From:   Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>
To:     Yibo Zhao <yiboz@codeaurora.org>
Cc:     make-wifi-fast@lists.bufferbloat.net,
        linux-wireless@vger.kernel.org, Felix Fietkau <nbd@nbd.name>,
        Rajkumar Manoharan <rmanohar@codeaurora.org>,
        Kan Yan <kyan@google.com>, linux-wireless-owner@vger.kernel.org
Subject: Re: [RFC/RFT] mac80211: Switch to a virtual time-based airtime scheduler
In-Reply-To: <89d32174b282006c8d4e7614657171be@codeaurora.org>
References: <20190215170512.31512-1-toke@redhat.com> <753b328855b85f960ceaf974194a7506@codeaurora.org> <87ftqy41ea.fsf@toke.dk> <acd33756e87ea64c608e306e5ee60aa2@codeaurora.org> <877ec2ykrh.fsf@toke.dk> <89d32174b282006c8d4e7614657171be@codeaurora.org>
Date:   Wed, 10 Apr 2019 12:40:01 +0200
Message-ID: <87a7gyw3cu.fsf@toke.dk>
MIME-Version: 1.0
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Sender: linux-wireless-owner@vger.kernel.org
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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).

> 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...

-Toke