linux-kernel - Re: iwlagn is getting very shaky

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Date:	Sun, 11 Dec 2011 21:56:47 +0200
From:	Emmanuel Grumbach <egrumbach@...il.com>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	Norbert Preining <preining@...ic.at>,
	"Guy, Wey-Yi" <wey-yi.w.guy@...el.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: iwlagn is getting very shaky

>> Could something be hogging the workqueues?
>>
>
>
> So I tried to understand what is going on with the workqueue and ended
> up to see that if we are lucky, we can need the workqueue for the BA
> handshake (could be AddBA / DelBA handling, or driver callback) while
> we are scanning. Which basically means that we will need to wait until
> the scan is over to handle these frames / callbacks. I got these
> measurements while stopping the BA session:
>
> * scanning working for roughly 3 seconds (pardon me not being precise,
> but with this order of magnitude I don't care much about the single
> millisecond..)
> * when scanning is over, the while loop in ieee80211_iface_work
> consumes 73 mgmt for about 34ms.
> ( how come we have so many beacons during those 3 seconds..., or maybe
> all the BCAST probe request ?, my network is quite busy...)
> * then the finally my stop_tx_ba_cb was served which took 10ms (time
> takes by the driver).
> * another series of beacons (10ms).

What about flushing the workqueue before we scan ?
This is not a bullet proof solution of course, we will still encounter
bad races, but at least  we would flush what we can before the
workqueue becomes unable for 4 seconds (!).
We can also delay the scan if we are in the middle of {add,del}BA
handshake, which is the only flow I can think about that needs
responsiveness. The other frame exchanges are MLME ones and involve
the wpa_supplicant (unless we are using the late WEXT). Hopefully the
wpa_supplicant won't request to scan in the middle of association or
so. There might be other features (mesh or whatever), that may be
hidden from the wpa_supplicant and require good responsiveness from
the wq too.
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