Re: [PATCH mac80211-next v6] mac80211: Switch to a virtual time-based airtime scheduler

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Felix Fietkau <nbd@nbd.name> writes:

> Hi Toke,
>
> Thanks for continuing to work on this! I just did a quick reading of the
> code and haven't tested this yet - I might have some more comments in
> the next few days.

Awesome! You're welcome, and thanks for taking a look :)

> On 2021-03-18 22:31, Toke Høiland-Jørgensen wrote:
>> This switches the airtime scheduler in mac80211 to use a virtual time-based
>> scheduler instead of the round-robin scheduler used before. This has a
>> couple of advantages:
>> 
>> - No need to sync up the round-robin scheduler in firmware/hardware with
>>   the round-robin airtime scheduler.
>> 
>> - If several stations are eligible for transmission we can schedule both of
>>   them; no need to hard-block the scheduling rotation until the head of the
>>   queue has used up its quantum.
>> 
>> - The check of whether a station is eligible for transmission becomes
>>   simpler (in ieee80211_txq_may_transmit()).
>> 
>> The drawback is that scheduling becomes slightly more expensive, as we need
>> to maintain an rbtree of TXQs sorted by virtual time. This means that
>> ieee80211_register_airtime() becomes O(logN) in the number of currently
>> scheduled TXQs because it can change the order of the scheduled stations.
>> We mitigate this overhead by only resorting when a station changes position
>> in the tree, and hopefully N rarely grows too big (it's only TXQs currently
>> backlogged, not all associated stations), so it shouldn't be too big of an
>> issue.
>> 
>> To prevent divisions in the fast path, we maintain both station sums and
>> pre-computed reciprocals of the sums. This turns the fast-path operation
>> into a multiplication, with divisions only happening as the number of
>> active stations change (to re-compute the current sum of all active station
>> weights). To prevent this re-computation of the reciprocal from happening
>> too frequently, we use a time-based notion of station activity, instead of
>> updating the weight every time a station gets scheduled or de-scheduled. As
>> queues can oscillate between empty and occupied quite frequently, this can
>> significantly cut down on the number of re-computations. It also has the
>> added benefit of making the station airtime calculation independent on
>> whether the queue happened to have drained at the time an airtime value was
>> accounted.
>> 
>> Co-developed-by: Yibo Zhao <yiboz@codeaurora.org>
>> Signed-off-by: Yibo Zhao <yiboz@codeaurora.org>
>> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
>> ---
>> Respinning this has taken way too long, but assuming anyone actually remembers
>> the previous version from a bit over a year ago, here's the changelog:
>> 
>> Changes since v5:
>>   Rebase on latest mac80211-next.
>> 
>>   Fix issue with scheduling hanging because the schedule position was not
>>   cleared when starting a new scheduling round.
>> 
>>   Switch the reciprocal calculation to use u32 (split 19/13) for per-station
>>   weights and a u64 only for the weight sum (to cut down on the number of 64-bit
>>   operations performed)
>> 
>>   Introduce the notion of time-based station activity when calculating weight
>>   sums. This also gets rid of the need for a "grace time" when catching up
>>   stations, since we now have a direct notion of when a station has been
>>   inactive for a while.
> Not sure if I'm misunderstanding the code, but this does not seem enough
> to me. From what I can see, you consider a station active if it has been
> scheduled in the last 100ms. Let's say we keep sending a single small
> packet to a particular sta every 90ms (thus keeping it active) for a
> long period of time and then suddenly start a really huge transfer.
> What keeps it from then taking an unreasonably large share of the
> airtime for as long as it takes for the virtual time to catch up?
>
> Am I missing something or should we maybe use the new notion of
> time-based activity *and* do a grace time catch up?

Hmm, yeah, I guess you're right? I haven't seen this in practice, but I
haven't been looking for it either. And I suppose someone who set out to
trigger this would be able to. So adding back the grace time (but maybe
with a longer interval?) would probably be best.

I'll wait for your other comments before respinning, though...

-Toke