linux-kernel - Re: Stop using tasklets for bottom halves

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Date:	Mon, 7 Sep 2009 17:14:06 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	"Luis R. Rodriguez" <mcgrof@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...e.hu>,
	Michael Buesch <mb@...sch.de>,
	"John W. Linville" <linville@...driver.com>,
	linux-wireless <linux-wireless@...r.kernel.org>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	Matt Smith <Matt.Smith@...eros.com>,
	Kevin Hayes <kevin@...eros.com>,
	Bob Copeland <me@...copeland.com>, Jouni Malinen <j@...fi>,
	Ivan Seskar <Seskar@...lab.rutgers.edu>, ic.felix@...il.com
Subject: Re: Stop using tasklets for bottom halves

On Mon, 7 Sep 2009 15:58:50 -0700
"Luis R. Rodriguez" <mcgrof@...il.com> wrote:

> A while ago I had read about an effort to consider removing tasklets
> [1] or at least trying to not use them. I'm unaware of the progress in
> this respect but since reading that article have always tried to
> evaluate whether or not we need tasklets on wireless drivers. I have
> also wondered whether work in irq context in other parts of the kernel
> can be moved to process context, a curious example being timers. I'll
> personally be trying to using only process context on bottom halves on
> future drivers but I figured it may be a good time to ask how serious
> was avoiding tasklets or using wrappers in the future to avoid irq
> context is or is it advised. Do we have a general agreement this is a
> good step forward to take? Has anyone made tests or changes on a
> specific driver from irq context to process context and proven there
> are no significant advantages of using irq context where you would
> have expected it?
> 
> Wireless in particular should IMHO not require taskets for anything
> time sensitive that I can think about except perhaps changing channels
> quickly and to do that appropriately also process pending RX frames
> prior to a switch. It remains to be seen experimentally whether or not
> using a workqueue for RX processing would affect the time to switch
> channels negatively but I doubt it would be significant. I hope to
> test that with ath9k_htc.
> 
> What about gigabit or 10 Gigabit Ethernet drivers ? Do they face any
> challenges which would yet need to be proven would not face issues
> when processing bottom halves in process context?
> 
> [1] http://lwn.net/Articles/239633/
> 
>   Luis
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Why not use NAPI, which is soft irq? Almost all 1G and 10G drivers
use NAPI.

Process context is too slow.

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