> In theory, this ought to produce a histogram of sojourn times (in
> microseconds):
> bpftrace -e 'kretprobe:codel_skb_time_func { @sojourn = lhist((nsecs - (retval << 10))/1000, 0, 100000, 1000); }'

Thanks for the tips!

> Can't get the CoDel drop mechanism to trigger on my system at all,
> though (a laptop running on iwl). I guess because there's queue
> backpressure to userspace first?

What's the tcp_congestion_control in your system? Maybe it is BBR that prevents bufferbloat.

> It would be interesting to see if it works for you, assuming you can get
> bpftrace to work on your test system :)

I can enable required kernel configuration easily, but cross-compile bpftrace for an ARM64 platform may take some time and effort.

On Thu, Nov 21, 2019 at 2:05 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:

Kan Yan <kyan@google.com> writes:

>> It would be interesting to get some samples of the actual sojourn time
>> as seen by CoDel in mac80211. Might be doable with bpftrace...
>
> I will try to add some trace event to get the sojourn time for the
> next round of tests.

In theory, this ought to produce a histogram of sojourn times (in
microseconds):

bpftrace -e 'kretprobe:codel_skb_time_func { @sojourn = lhist((nsecs - (retval << 10))/1000, 0, 100000, 1000); }'

Can't get the CoDel drop mechanism to trigger on my system at all,
though (a laptop running on iwl). I guess because there's queue
backpressure to userspace first?

It would be interesting to see if it works for you, assuming you can get
bpftrace to work on your test system :)

-Toke