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Subject: Re: [PATCH] mac80211: rework locking for txq scheduling / airtime
 fairness
To:     Yibo Zhao <yiboz@codeaurora.org>
Cc:     linux-wireless@vger.kernel.org, johannes@sipsolutions.net,
        =?UTF-8?Q?Toke_H=c3=b8iland-J=c3=b8rgensen?= <toke@redhat.com>
References: <20190315100335.91445-1-nbd@nbd.name>
 <b2bded992cb02bad84fc738b7e4b1837@codeaurora.org>
From:   Felix Fietkau <nbd@nbd.name>
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Date:   Thu, 10 Oct 2019 07:56:32 +0200
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On 2019-10-10 04:43, Yibo Zhao wrote:
> Hi Felix,
> 
> In ath10k, we might have situations like,
> 
>            CPU0                           CPU1
> ...                                   ieee80211_txq_schedule_start()
> ...                                   while(ieee80211_next_txq()) {
> ...                                       driver dequeue packets from 
> txq
> ...                                       ieee80211_return_txq()
> ieee80211_txq_schedule_start()            ...
> ieee80211_next_txq()                  }
> driver dequeue packets from txq       ieee80211_txq_schedule_end()
> ieee80211_return_txq()
> ieee80211_txq_schedule_end()
> 
> The problem is while CPU1 is looping to dequeue txqs, the schedule_round 
> which is used to make sure no infinite loop will be changed in CPU0. So 
> a txq has already been scheduled will be scheduled again then the loop 
> won't be end at that point as our expected.
Hi,

I think this is something that should be fixed in the driver. The
comment for ieee80211_txq_schedule_start() already states:
"The driver must not call multiple TXQ scheduling rounds concurrently"

The way that I'm dealing with this in mt76 is that I ensure that all txq
scheduling is done from a single tasklet, which is scheduled from the tx
status and the queue wake path.

I think that approach should perform well in ath10k as well, and it will
avoid the cost of waiting for a lock on one CPU while the other is
scheduling tx already.

- Felix