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

[Felix Fietkau]

On 2019-03-08 12:05, Toke Høiland-Jørgensen wrote:
> Felix Fietkau <nbd at nbd.name> writes:
> 
>> On 2019-02-15 18:05, 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.
>>> 
>>> Signed-off-by: Toke Høiland-Jørgensen <toke at redhat.com>
>> The approach looks good to me, but I haven't really reviewed it very
>> carefully yet. Just some points that I noticed below:
> 
> Cool!
> 
>>> diff --git a/net/mac80211/sta_info.c b/net/mac80211/sta_info.c
>>> index 11f058987a54..9d01fdd86e2d 100644
>>> --- a/net/mac80211/sta_info.c
>>> +++ b/net/mac80211/sta_info.c
>>> @@ -389,7 +389,6 @@ struct sta_info *sta_info_alloc(struct ieee80211_sub_if_data *sdata,
>>>  	for (i = 0; i < IEEE80211_NUM_ACS; i++) {
>>>  		skb_queue_head_init(&sta->ps_tx_buf[i]);
>>>  		skb_queue_head_init(&sta->tx_filtered[i]);
>>> -		sta->airtime[i].deficit = sta->airtime_weight;
>>>  	}
>>>  
>>>  	for (i = 0; i < IEEE80211_NUM_TIDS; i++)
>>> @@ -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;
>>> +	sta->airtime[ac].v_t += airtime / sta->airtime_weight;
>>> +	ieee80211_resort_txq(&local->hw, txq);
>> These divisions could be a bit expensive, any way to change the
>> calculation to avoid them?
> 
> Yeah, given that the denominators are constant from the PoV of the fast
> path, we can pre-compute reciprocals and turn these divides into
> multiplications. Will incorporate that...
Sounds good.

>> I'm a bit worried about this part. Does that mean that vif txqs always
>> have priority over sta txqs?
> 
> Yeah, it does. This sort of mirrors what the existing airtime scheduler
> does (because VIFs don't have an airtime deficit), but because it's a
> round-robin scheduler the effect is less severe as long as there are
> stations able to transmit.
> 
> I guess the obvious fix is to start accounting airtime usage for the VIF
> as well? We may want to do that in any case, as that would also give
> users a convenient way to set policy for multicast traffic. Any
> objections to this?
I think this is a good idea.

- Felix