Re: [PATCH] mac80211: add stop/start logic for software TXQs

[Johannes Berg <johannes@sipsolutions.net>]

On Tue, 2018-06-26 at 12:46 +0530, Manikanta Pubbisetty wrote:
> 
> -	if (test_bit(IEEE80211_TXQ_STOP, &txqi->flags))
> +	if (test_bit(IEEE80211_TXQ_STOP, &txqi->flags) ||
> +	    test_bit(IEEE80211_TXQ_PAUSED, &txqi->flags))
>  		goto out;
>  
> +	if (local->txqs_stopped) {

How is it even possible to have txqs_stopped set, but not TXQ_PAUSED? It
 seemed to me - from a first glance at the rest of the code - that the
txqs_stopped is just tracking when you can re-enable, but you propagate
it to the TXQs?

> +		set_bit(IEEE80211_TXQ_PAUSED, &txqi->flags);

Oh. Or maybe I just misread that and you're only "lazily" propagating it
here?

> +		if (txq->sta) {
> +			sta = container_of(txq->sta, struct sta_info, sta);
> +			atomic_set(&sta->txqs_paused, 1);
> +		}

It seems to me you could remove this - possibly at the expense of doing
a little more work in ieee80211_wake_txqs()?

Have you measured it and found it to be a problem?

The atomic stuff here doesn't work the way you want it to though. In
fact, even the lazy propagation doesn't work the way you want it to, I
think!

Thinking about it:

CPU 1                           CPU 2
 * you're disabling TX
   -> local->txqs_stopped = 1
                                 * check local->txqs_stopped
 * re-enable TX
   -> local->txqs_stopped = 0
 * call ieee80211_wake_txqs()
   check sta->txqs_paused
                                 * set txq->flags |= PAUSED
                                 * set sta->txqs_paused

I don't see how you can prevent this right now? In
ieee80211_tx_dequeue() you have the fq lock, but you're not taking it on
the other side (in __ieee80211_stop_queue/__ieee80211_wake_queue).

If you always iterate the stas/TXQs and set the PAUSED bit non-lazily at
least you have a single source of truth. You may still need the
txqs_stopped bitmap, but only in stop_queue/wake_queue which has its own
locking.


> +		for (i = 0; i < ARRAY_SIZE(sta->sta.txq); i++) {
> +			txqi = to_txq_info(sta->sta.txq[i]);
> +
> +			if (!test_and_clear_bit(IEEE80211_TXQ_PAUSED,
> +						&txqi->flags))
> +				continue;
> +
> +			drv_wake_tx_queue(local, txqi);

Maybe that should be conditional on there being packets? Actually, no
... ok, the lazy setting helps you with this because it means that the
driver wanted a packet but didn't get one, so now you should wake it. If
it never wanted a packet, then you don't care about the state.

Ok - so that means we need to keep the lazy setting, but fix the locking
:-)

> +		/* Software interfaces like AP_VLAN, monitor do not have
> +		 * vif specific queues.
> +		 */
> +		if (!sdata->dev || !vif->txq)
> +			continue;

Is there any way you can have vif->txq && !sdata->dev? It seems to me
that no, and then you only need the vif->txq check?

> @@ -298,10 +354,15 @@ static void __ieee80211_wake_queue(struct ieee80211_hw *hw, int queue,
>  	if (local->q_stop_reasons[queue][reason] == 0)
>  		__clear_bit(reason, &local->queue_stop_reasons[queue]);
>  
> +	if (local->ops->wake_tx_queue)
> +		__clear_bit(reason, &local->txqs_stopped);
> +
>  	if (local->queue_stop_reasons[queue] != 0)
>  		/* someone still has this queue stopped */
>  		return;
>  
> +	ieee80211_wake_txqs(local);

I'm not sure this is the right place to hook in?

This might interact really strangely with drivers - which also get here.
Perhaps we should make this conditional on reason != DRIVER?

Also, your bitmap setting should be different - it should be per queue?
But then since "queue" here should basically be an AC for iTXQ drivers,
I think you need to handle that properly.

Right now you get bugs if you do

stop_queue(0, reason=aggregation)
stop_queue(1, reason=aggregation)
wake_queue(0, reason=aggregation)

-> TXQs will wake up because local->txqs_stopped is only stopping the
reason, not the queue number.


So to summarize:
 * overall the approach seems fine, and the lazy disabling is probably
   for the better, but
 * need to address locking issues with that, and
 * need to address the refcounting issues.


johannes