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

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

Yibo Zhao <yiboz@codeaurora.org> writes:

> On 2019-02-16 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.

>> +	sta->airtime[ac].v_t += airtime / sta->airtime_weight;
> Another question. Any plan for taking v_t overflow situation into 
> consideration? u64 might be enough for low throughput products but not 
> sure for high end products. Something like below for reference:

The unit for the variable is time, not bytes, so it is unaffected by
throughput. 2**64 microseconds is 584554 *years* according to my
'units' binary, so don't think we have to worry too much about this
overflowing ;)

-Toke