On 2019-03-08 12:05, Toke Høiland-Jørgensen wrote:
> Felix Fietkau <nbd@xxxxxxxx> 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@xxxxxxxxxx>
>> 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
