Re: [ath9k-devel] [RFC] ath9k: Detect and work-around tx-queue hang.

From: Felix Fietkau <nbd@openwrt.org>
To: Ben Greear <greearb@candelatech.com>
Cc: Sujith Manoharan <sujith@msujith.org>,
	ath9k-devel@venema.h4ckr.net, linux-wireless@vger.kernel.org
Subject: Re: [ath9k-devel] [RFC] ath9k: Detect and work-around tx-queue hang.
Date: Wed, 13 Mar 2013 15:18:24 +0100	[thread overview]
Message-ID: <51408AB0.7000600@openwrt.org> (raw)
In-Reply-To: <513F70E5.2050004@candelatech.com>

On 2013-03-12 7:16 PM, Ben Greear wrote:
> On 02/22/2013 04:55 AM, Ben Greear wrote:
>> On 02/22/2013 04:38 AM, Felix Fietkau wrote:
>>> On 2013-02-22 1:25 PM, Sujith Manoharan wrote:
>>>> Felix Fietkau wrote:
>>>>> Please also check if the station(s) that the frames are queued for are
>>>>> in powersave state for some reason. That would prevent the tx path from
>>>>> throwing them in the hw queue, yet they'd still take up pending-frame
>>>>> slots. I was planning on fixing this eventually by expiring frames that
>>>>> stay in the queue for too long, but haven't decided on the exact
>>>>> approach yet.
>>>>
>>>> PS is disabled for multi-VIF.
>>> What about off-channel PS due to scans, etc.
>>
>> Scan is always locked to the same channel in this setup (once
>> a single station is associated).
>>
>> The stations stay associated while this problem happens (the
>> high-priority queue seems to work just fine, which may be
>> the reason they stay associated just fine.)
>>
>> In some cases, I see packets delivered around 30 seconds late...
>> aside from PS and off-channel..any idea what could make a packet
>> stick around that long in the tx queues?
> 
> One of our customers on a 3.5.7+ kernel hit the problem without
> using any RF attenuator...just over-the-air communication to
> their AP.  It happened on both 2.4 and 5Ghz bands.  Seems rx
> signal is around 40 in their environment. It took them around
> 24 hours to hit the problem on average.
> 
> Last we checked, we could fairly easily reproduce this in
> our lab using an attenuator and a certain setup, so if there
> is any debugging we could add to help narrow down what
> might be causing this, we can give that a try.
> 
> For instance, is there any good way to know for certain
> if packets in the queue are in power-save or not?  I know
> we at least attempt to disable power-save, but possibly
> it gets re-enabled somehow?
The ath_node struct tracks if a node is sleeping or not. If a node is
sleeping, its tid queues can still hold some frames but will not be
serviced by ath_txq_schedule.

- Felix