[Make-wifi-fast] [PATCH RFC/RFT 4/4] mac80211: Apply Airtime-based Queue Limit (AQL) on packet dequeue

[Felix Fietkau]

On 2019-09-19 14:22, Toke Høiland-Jørgensen wrote:
> From: Toke Høiland-Jørgensen <toke at redhat.com>
> 
> Some devices have deep buffers in firmware and/or hardware which prevents
> the FQ structure in mac80211 from effectively limiting bufferbloat on the
> link. For Ethernet devices we have BQL to limit the lower-level queues, but
> this cannot be applied to mac80211 because transmit rates can vary wildly
> between packets depending on which station we are transmitting it to.
> 
> To overcome this, we can use airtime-based queue limiting (AQL), where we
> estimate the transmission time for each packet before dequeueing it, and
> use that to limit the amount of data in-flight to the hardware. This idea
> was originally implemented as part of the out-of-tree airtime fairness
> patch to ath10k[0] in chromiumos.
> 
> This patch ports that idea over to mac80211. The basic idea is simple
> enough: Whenever we dequeue a packet from the TXQs and send it to the
> driver, we estimate its airtime usage, based on the last recorded TX rate
> of the station that packet is destined for. We keep a running per-AC total
> of airtime queued for the whole device, and when that total climbs above 8
> ms' worth of data (corresponding to two maximum-sized aggregates), we
> simply throttle the queues until it drops down again.
> 
> The estimated airtime for each skb is stored in the tx_info, so we can
> subtract the same amount from the running total when the skb is freed or
> recycled. The throttling mechanism relies on this accounting to be
> accurate (i.e., that we are not freeing skbs without subtracting any
> airtime they were accounted for), so we put the subtraction into
> ieee80211_report_used_skb().
> 
> This patch does *not* include any mechanism to wake a throttled TXQ again,
> on the assumption that this will happen anyway as a side effect of whatever
> freed the skb (most commonly a TX completion).
> 
> The throttling mechanism only kicks in if the queued airtime total goes
> above the limit. Since mac80211 calculates the time based on the reported
> last_tx_time from the driver, the whole throttling mechanism only kicks in
> for drivers that actually report this value. With the exception of
> multicast, where we always calculate an estimated tx time on the assumption
> that multicast is transmitted at the lowest (6 Mbps) rate.
> 
> The throttling added in this patch is in addition to any throttling already
> performed by the airtime fairness mechanism, and in principle the two
> mechanisms are orthogonal (and currently also uses two different sources of
> airtime). In the future, we could amend this, using the airtime estimates
> calculated by this mechanism as a fallback input to the airtime fairness
> scheduler, to enable airtime fairness even on drivers that don't have a
> hardware source of airtime usage for each station.
> 
> [0] https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/588190/13/drivers/net/wireless-4.2/ath/ath10k/mac.c#3845
One thing that might be missing here is dealing with airtime accounting
of frames that remain queued in the driver/hardware because the station
is in powersave mode.

- Felix