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From:   Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>
To:     Kan Yan <kyan@google.com>
Cc:     Dave Taht <dave.taht@gmail.com>,
        Rajkumar Manoharan <rmanohar@codeaurora.org>,
        Kevin Hayes <kevinhayes@google.com>,
        Make-Wifi-fast <make-wifi-fast@lists.bufferbloat.net>,
        linux-wireless <linux-wireless@vger.kernel.org>,
        Yibo Zhao <yiboz@codeaurora.org>,
        John Crispin <john@phrozen.org>,
        Johannes Berg <johannes@sipsolutions.net>,
        Lorenzo Bianconi <lorenzo@kernel.org>,
        Felix Fietkau <nbd@nbd.name>
Subject: Re: [Make-wifi-fast] [PATCH v8 0/2] Implement Airtime-based Queue Limit (AQL)
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Date:   Fri, 22 Nov 2019 11:45:50 +0100
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Kan Yan <kyan@google.com> writes:

>> In theory, this ought to produce a histogram of sojourn times (in
>> microseconds):
>> bpftrace -e 'kretprobe:codel_skb_time_func { @sojourn =3D 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's not BBR, just plain old CUBIC. I've seen the issue before that it's
almost impossible to build a queue in the mac80211 layer when the TCP
session is originated on the local machine, though...

>> 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.

Yeah, bpftrace can be a bit of a pain to get running; but it may be
worth the investment longer term as well. It really is quite useful! :)

Some links:

Install guide:
https://github.com/iovisor/bpftrace/blob/master/INSTALL.md

Tutorial by one-liners:
https://github.com/iovisor/bpftrace/blob/master/docs/tutorial_one_liners.md

Reference guide:
https://github.com/iovisor/bpftrace/blob/master/docs/reference_guide.md#5-t=
racepoint-static-tracing-kernel-level

-Toke