Rajkumar Manoharan <rmanohar@xxxxxxxxxxxxxx> writes:
> On 2018-10-10 04:15, Toke Høiland-Jørgensen wrote:
>> Rajkumar Manoharan <rmanohar@xxxxxxxxxxxxxx> writes:
>>
>>> On 2018-10-09 05:32, Toke Høiland-Jørgensen wrote:
>>>> This adds airtime accounting and scheduling to the mac80211 TXQ
>>>> scheduler. A new callback, ieee80211_sta_register_airtime(), is added
>>>> that drivers can call to report airtime usage for stations.
>>>>
>>>> When airtime information is present, mac80211 will schedule TXQs
>>>> (through ieee80211_next_txq()) in a way that enforces airtime
>>>> fairness
>>>> between active stations. This scheduling works the same way as the
>>>> ath9k
>>>> in-driver airtime fairness scheduling. If no airtime usage is
>>>> reported
>>>> by the driver, the scheduler will default to round-robin scheduling.
>>>>
>>>> For drivers that don't control TXQ scheduling in software, a new API
>>>> function, ieee80211_txq_may_transmit(), is added which the driver can
>>>> use
>>>> to check if the TXQ is eligible for transmission, or should be
>>>> throttled to
>>>> enforce fairness. Calls to this function must also be enclosed in
>>>> ieee80211_txq_schedule_{start,end}() calls to ensure proper locking.
>>>> TXQs
>>>> that are throttled by ieee802111_txq_may_transmit() will be woken up
>>>> again
>>>> by a check added to the ieee80211_wake_txqs() tasklet.
>>>>
>>>
>>> Toke,
>>>
>>> I am observing soft lockup issues again with this new series while
>>> running traffic with 50 clients. I am continuing testing with earlier
>>> series along with snippet I shared.
>>
>> Are these new lockups (that was not in your patched previous version),
>> or did I just not get all your lock-related fixes incorporated?
>>
>>> When driver operates in pull-mode, throttled txqs are marked and
>>> refilled in airtime_tasklet. This is causing major throughput drops
>>> and packet loss and I am suspecting the latency in replenishing
>>> deficit. Whereas in push-mode or in ath9k model, refill happens
>>> quicker at every packet indication as well as tx completion.
>>
>> Yeah, the tasklet shouldn't be the main source of deficit replenishing.
>> Can see why that would give bad performance :)
>>
>>> I am planning to get rid of tasklet completely as it is only meant for
>>> pull-mode. It would be better to refill in may_transmit() itself.
>>
>> Hmm, right. So the way to do this correctly (from a fairness point of
>> view) would be something like this (in max_tx()):
>>
>> if (this_txq.stn.deficit > 0)
>> return true;
>>
>> else if (any queued TXQ currently have positive deficit)
>> return false; /* other TXQ should try may_tx() later and get
>> permission */
>>
>> else /* all deficits < 0 */
>> return replenish_deficits(this_txq);
>>
>> And replenish_deficits() would be something like:
>>
>> replenish_deficits(this_txq) {
>> repeat:
>> for (txq in queued txqs) {
>> txq.stn.deficit += stn.weight;
>> if (txq.stn.deficit > 0 && !wake_txq)
>> wake_txq = txq;
>> }
>> if not wake_txq:
>> goto repeat;
>>
>> if (this_txq.stn.deficit > 0)
>> return true;
>> else
>> drv_wake_tx_queue(wake_txq);
>> }
>>
>> The wake_tx_queue call may have to be delegated to a tasklet still, to
>> avoid the infinite recursion problem I mentioned earlier. But the
>> tasklet could be made simpler and wouldn't have to be called so
>> often...
>>
>> Does the above make sense?
>>
> Hmm... mine is bit different. txqs are refilled only once for all txqs.
> It will give more opportunity for non-served txqs. drv_wake_tx_queue
> won't be
> called from may_tx as the driver anyway will not push packets in
> pull-mode.
So, as far as I can tell, this requires the hardware to "keep trying"?
I.e., if it just stops scheduling a TXQ after may_transmit() returns
false, there is no guarantee that that TXQ will ever get re-awoken
unless a new packet arrives for it?
-Toke
