On Sun, Feb 7, 2016 at 12:06 PM, Emmanuel Grumbach <egrumbach@xxxxxxxxx> wrote:
> On Sun, Feb 7, 2016 at 11:08 AM, Felix Fietkau <nbd@xxxxxxxxxxx> wrote:
>> On 2016-02-07 08:25, Emmanuel Grumbach wrote:
>>> On Fri, Feb 5, 2016 at 12:41 PM, Felix Fietkau <nbd@xxxxxxxxxxx> wrote:
>>>> Requires software tx queueing support. frag_list support (for zero-copy)
>>>> is optional.
>>>>
>>>> Signed-off-by: Felix Fietkau <nbd@xxxxxxxxxxx>
>>>
>>> Looks nice!
>>> This would allow us to create aggregates of TCP Acks, the problem is
>>> that when you are mostly receiving data, the hardware queues are
>>> pretty much empty (nothing besides the TCP Acks which should go out
>>> quickly) so that packets don't pile up in the software queues and
>>> hence you don't have enough material to build an A-MSDU.
>>> I guess that for AP oriented devices, this is ideal solution since you
>>> can't rely on TSO (packets are not locally generated) and this allows
>>> to build an A-MSDU without adding more latency since you build an
>>> A-MSDU with packets that are already in the queue waiting instead of
>>> delaying transmission of the first packet.
>>> IIRC, the latter was the approach chose by the new Marvell driver
>>> posted a few weeks ago. This approach is better in my eyes.
>>> For iwlwifi which is much more station oriented (of GO which is
>>> basically an AP with locally generated traffic), I took the TSO
>>> approach. I guess we could try to change iwlwifi to use your tx
>>> queues, and check how that works. This would allow us to have A-MSDU
>>> on bridged traffic as well, although this use case is much less common
>>> for Intel devices.
>
>
>> Can the iwlwifi firmware maintain per-sta per-tid queues? Because that
>> way you would get the most benefits from using that tx queueing
>> infrastructure.
>
> Well... iwlwifi and athXk are very different. iwlwifi really has the
> concept of queues and not flat descriptors.
> Any Tx descriptor lives in the context of a Tx queue which is
> per-sta-per tid when A-MPDUs is enabled.
> We are now moving to a scheme in which we will have per-sta-per-tid
> queue regardless of the A-MPDU state which will make much sense to tie
> to the tx queueing infrastructure.
> Thing is that in that case I am afraid we will not have enough packets
> in the software tx queue to get A-MSDUs from your code. with TSO, it
> is easier :) Still worth trying to work with this instead of TSO and
> see how it goes. That won't happen anytime soon though.
>
>>
>>>> +
>>>> +static bool ieee80211_amsdu_aggregate(struct ieee80211_sub_if_data *sdata,
>>>> + struct sta_info *sta,
>>>> + struct ieee80211_fast_tx *fast_tx,
>>>> + struct sk_buff *skb)
>>>> +{
>>>> + struct ieee80211_local *local = sdata->local;
>>>> + u8 tid = skb->priority & IEEE80211_QOS_CTL_TAG1D_MASK;
>>>> + struct ieee80211_txq *txq = sta->sta.txq[tid];
>>>> + struct txq_info *txqi;
>>>> + struct sk_buff **frag_tail, *head;
>>>> + int subframe_len = skb->len - ETH_ALEN;
>>>> + int max_amsdu_len;
>>>> + __be16 len;
>>>> + void *data;
>>>> + bool ret = false;
>>>> + int n = 1;
>>>> +
>>>> + if (!ieee80211_hw_check(&local->hw, TX_AMSDU))
>>>> + return false;
>>>> +
>>>> + if (!txq)
>>>> + return false;
>>>> +
>>>> + txqi = to_txq_info(txq);
>>>> + if (test_bit(IEEE80211_TXQ_NO_AMSDU, &txqi->flags))
>>>> + return false;
>>>> +
>>>> + /*
>>>> + * A-MPDU limits maximum MPDU size to 4095 bytes. Since aggregation
>>>> + * sessions are started/stopped without txq flush, use the limit here
>>>> + * to avoid having to de-aggregate later.
>>>> + */
>>>> + max_amsdu_len = min_t(int, sta->sta.max_amsdu_len, 4095);
>>>
>>> So you can't get 10K A-MSDUs? I don't see where you check that you
>>> have an A-MPDU session here. You seem to be applying the 4095 limit
>>> also for streams that are not an A-MPDU?
>>> I guess you could check if the sta is a VHT peer, in that case, no
>>> limit applies.
>> The explanation for the missing A-MPDU change is in that comment -
>> checking for an active A-MPDU session would make it unnecessarily complex.
>> Good point about checking for VHT capabilities to remove this limit, I
>> will add that.
Yes - I read the comment, but it seemed very sub-optimal to limit all
the A-MSDUs to 4K. With TSO I can get up to 10K and it really helps
TPT.
One more point. In VHT, there may be a limit on the numbers of
subframes in the A-MSDU. I don't see you handle that. Maybe I missed
it?
And... in case the driver doesn't handle frag_list, you linearize the
skb which is pretty much the only thing you can do at this stage. But,
when you'll lift the 4095 bytes limit, you'll get 11K A-MSDU,
linarizing such a long packet is really putting the memory manager
under pressure. This is an order 4 allocation, for each A-MSDU. Note
that iwlwifi (and probably other drivers) can handle gather DMA in Tx,
but they have a limited number of frags they can handle. iwlwifi e.g.
can handle up to 20 frags, but 3 are taken for "paperwork". You'll
have 2 frags per subframe at least (assuming that each subframe's
payload is nicely contiguous and not on a page boundary). I think that
it may be worthwhile to ask the driver how many frags it is supposed
to handle. I can't promise iwlwifi will use it, but I guess it will be
useful for someone.
>>
>> - Felix
--
To unsubscribe from this list: send the line "unsubscribe linux-wireless" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
