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

Toke Høiland-Jørgensen toke at toke.dk
Tue May 16 06:17:32 EDT 2017

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

> 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.

Right. So how does this differ from using a low-prio CC (like LEDBAT) on
each of the MPTCP subflows? (If that makes sense; my knowledge of MPTCP
is somewhat limited).

>>> 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?

Yeah, that's what I meant with it being a private variable. What you can
do is add an accessor function somewhere in mac80211, which would take a
struct net_device and return the queue backlog.

Something like

u32 mac80211_get_backlog(struct net_device *dev)
	struct ieee80211_local *local = wdev_priv(dev->ieee80211_ptr);
        return local->fq->backlog;

> Thank you for your help.

You're welcome :)


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