[Make-wifi-fast] Get hardware queue length for wireless interface in linux kernel

Mohan, Nitinder nitinder.mohan at helsinki.fi
Tue May 16 05:33:13 EDT 2017

Hi Toke,

Thank you for your quick reply.

"Mohan, Nitinder" <nitinder.mohan at helsinki.fi> writes:

> Hi,
> I am a Ph.D. student working on a bufferbloat resistant scheduler for
> Multi-Path TCP. I was unsure of which list my question would be more
> suitable in so I am sending this in both make-wifi-fast and bloat-dev
> list.

Sounds interesting! What, exactly, is such a scheduler going to be
doing? :)

The MPTCP scheduler takes into account lower layer buffer levels and schedules traffic over the interface to avoid HW bufferbloat. The simulation testing gave us quite good results. Now we wanted to implement the algorithm on linux kernel to evaluate our approach. As I am not very well-versed with linux kernel, I was having some problems implementing this.

> While implementing the scheduler in linux kernel, I was unable to get
> the current number of bytes in hardware queue for wlan interface. As
> currently, linux does not employ BQL for wifi devices, I could not get
> this value from dql->num_queued defined in dynamic_queue_limits.h. I
> also tried to get queue length from Queueing discipline structure i.e.
> qdisc->qstats.qlen defined in sch_generic.h yet it still gives me a
> zero value (I am getting zero values for other parameters in qstat as
> well so I am sure it is not because the queue length never becomes
> more than 0).
> If you have any idea for getting queue length for wireless interfaces,
> please do reply. Any help would be highly appreciated.

There's no general interface to get the queue length for WiFi
interfaces. If you're using a driver that is using the mac80211
intermediate queues (i.e., ath9k, ath10k or mt76), there are
fq->backlock and fq->memory_usage which live inside the struct fq in
struct ieee80211_local. However, these are private state vars inside
mac80211, so not immediately accessible from other parts of the kernel.
You could add an access function to mac80211, though.

I am using ath9k drivers so fq->backlog would work well for me. I was giving the mac80211 structure a look but cant seem to figure out an access point to it (say from netdevice.h). Do you have an idea of how to retrieve a struct in mac80211 that could point me to struct fq?

Other drivers will have their own queueing implementations, that you
probably can't get at. Some devices will even have most of their
queueing in firmware.


Thank you for your help.


Nitinder Mohan
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