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Subject: Re: [PATCH v11 4/4] mac80211: Use Airtime-based Queue Limits (AQL) on
 packet dequeue
To:     Kan Yan <kyan@google.com>, johannes@sipsolutions.net
Cc:     linux-wireless@vger.kernel.org,
        make-wifi-fast@lists.bufferbloat.net, toke@redhat.com,
        yiboz@codeaurora.org, john@phrozen.org, lorenzo@kernel.org,
        rmanohar@codeaurora.org, kevinhayes@google.com
References: <20191119060610.76681-1-kyan@google.com>
 <20191119060610.76681-5-kyan@google.com>
From:   Felix Fietkau <nbd@nbd.name>
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Date:   Wed, 26 Feb 2020 21:30:34 +0100
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Hi,

I've take a closer look at the AQL implementation, and I found some
corner cases that need to be addressed soon:

- AQL estimated airtime does not take into account A-MPDU, so it is
significantly overestimating airtime use for aggregated traffic,
especially on high rates.
My proposed solution would be to check for a running aggregation session
and set estimated tx time to something like:
expected_airtime(16 * skb->len) / 16.

- We need an API that allows the driver to change the pending airtime
values, e.g. subtract estimated tx time for a packet.
mt76 an ath9k can queue packets inside the driver that are not currently
in the hardware queues. Typically if the txqs have more data than what
gets put into the hardware queue, both drivers will pull an extra frame
and queue it in its private txq struct. This frame will get used on the
next txq scheduling round for that particular station.
If you have lots of stations doing traffic (or having driver buffered
frames in powersave mode), this could use up a sizable chunk of the AQL
budget.
While removing the airtime of those packages would lead to AQL
temporarily underestimating airtime, I think it would be better than
overestimating it.

I will work on some patches. What do you think about these issues and my
proposed fixes?

- Felix