Re: [PATCH 0/3] RFC: prioritizing data over HCI

[Mat Martineau <mathewm@codeaurora.org>]

On Tue, 9 Aug 2011, Gustavo Padovan wrote:

> * Mat Martineau <mathewm@codeaurora.org> [2011-08-08 16:29:51 -0700]:
>
>>
>> On Fri, 5 Aug 2011, Gustavo Padovan wrote:
>>
>>> * Peter Hurley <peter@hurleysoftware.com> [2011-08-04 19:09:37 -0400]:
>>>
>>>> Hi Mat,
>>>>
>>>> On Thu, 2011-08-04 at 13:37 -0400, Mat Martineau wrote:
>>>>
>>>>> I had a recent discussion with Gustavo about HCI queuing issues with
>>>>> ERTM:
>>>>>
>>>>> http://www.spinics.net/lists/linux-bluetooth/msg13774.html
>>>>>
>>>>> My proposal is to move tx queuing up to L2CAP, and have the HCI tx
>>>>> task only handle scheduling.  Senders would tell HCI they have data to
>>>>> send, and HCI would call back to pull data.  I've been focused on
>>>>> L2CAP - it would be possible to make a similar queuing change to
>>>>> SCO/eSCO/LE, but not strictly necessary.
>>>>
>>>> Would you please clarify this approach (perhaps in a separate thread)?
>>>>
>>>> For example, how does having tx queues in l2cap_chan (instead of the
>>>> hci_conn) solve the latency problems in ERTM when replying to
>>>> REJ/SREJ/poll? Won't there potentially be just as much data already
>>>> queued up? Is the plan to move the reply to the front of the tx queue
>>>> because reqseq won't need to be assigned until the frame is actually
>>>> pulled off the queue?
>>>
>>> Exactly. ERTM connections can get dropped if the too much data is buffered and
>>> we need to send final bit for example.
>>
>> Right now, an outgoing ERTM frame goes through two queues: a
>> channel-specific ERTM tx queue and the HCI ACL data_q.  The ERTM
>> control field is not constructed until a frame is removed from the
>> ERTM tx queue and pushed to the HCI data_q, so the s-frame latency
>> problem comes in when the the HCI data_q gets deep.  S-frames are
>> already pushed directly in to the HCI data_q, bypassing the data tx
>> queue.
>>
>> From an ERTM perspective, the goal is to defer assignment of reqseq
>> and f-bit values as late as possible, so the remote device gets the
>> most recent information on data frames and polls that have been
>> received.  The optimal thing to do (by this measurement, anyway) is
>> to build the ERTM control field as data is sent to the baseband --
>> in other words, to eliminate the HCI data_q altogether.
>>
>> (Yeah, without the data_q, ERTM would need additional queues for
>> s-frames and retransmitted i-frames)
>>
>> So, without a data_q, what makes sense?  If there are ACL buffers
>> available and no pending L2CAP senders, it would be great to push
>> data straight out to the baseband.  If we're blocked waiting for
>> num_completed_packets, then receipt of num_completed_packets is the
>> natural time to pull data from the tx queues that now happen to be
>> up in the L2CAP layer.
>>
>> There are certainly locking, task scheduling, data scheduling, QoS,
>> and efficiency issues to consider.  This is just a general
>> description for now, and I'm trying to see if there's enough
>> interest (or few enough obvious gotchas) to put some serious effort
>> in to moving forward.
>
> Getting rid of conn->data_q makes sense. I started a patch to create the
> struct hci_chan that Luiz proposed. It would be one HCI channel per L2CAP
> connection. The buffer (acl_cnt) would be now divided by the number of
> channels and not the number of connections. This is a first step to support
> QoS and priority inside ERTM. QoS then would just need new scheduler rules.
>
> +struct hci_chan {
> +       struct list_head list;
> +       struct hci_conn *conn;
> +       struct sk_buff_head data_q;
> +       unsigned int    sent;
> +}

At the BlueZ summit last year, the group settled on using an "HCI 
channel" as an abstraction for AMP logical links.  We have a hci_chan 
struct already that you could add to.  Before making changes to HCI 
data structures, could we first work on upstreaming the AMP HCI 
changes (which also include some QoS-related code)?


> So in the next step for ERTM we move the queue to L2CAP and create a callback
> to call from HCI at the moment of push data to the baseband. The function in
> L2CAP would set the last control bits in the first packet of the queue and
> sent it through.

This actually causes some problems for ERTM, since skbs are cloned 
before they are pushed to HCI.  skb data is not supposed to be 
modified after cloning.

If there's a callback to L2CAP anyway, why not have L2CAP provide the 
skb at that time instead of modifying data it provided earlier?


> Then the queue can be split in two by adding a pointer that will mark which
> element divides the queue between prio and normal. New prio skbs would just be
> queued after this element and before the rest.

I think it's simpler and less bug-prone to just have two queues. 
Either way, it's one more pointer.

However, I'm still not sure we want any queues in hci_chan.  It's not 
very complicated to have the queue in the L2CAP layer, and gives ERTM 
the control it needs.


> I still need to think on locking here. (and also finish my patches 
> that move all the bluetooth to workqueue)

Keep in mind that skb_queue() and skb_dequeue() have their own 
locking. Non-ERTM modes shouldn't need locking when HCI calls back for 
skbs.

For ERTM, we need to figure out a good way to protect ERTM state (like 
buffer_seq and next_tx_seq) without using the socket lock.

--
Mat Martineau
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum