netdev - Re: Stop using tasklets for bottom halves

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Message-Id: <1252376254.21261.2052.camel@gandalf.stny.rr.com>
Date:	Mon, 07 Sep 2009 22:17:34 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	"Luis R. Rodriguez" <mcgrof@...il.com>,
	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, 2009-09-07 at 17:14 -0700, Stephen Hemminger wrote:
> 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
> > --
> > To unsubscribe from this list: send the line "unsubscribe netdev" in
> > the body of a message to majordomo@...r.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 
> Why not use NAPI, which is soft irq? Almost all 1G and 10G drivers
> use NAPI.
> 
> Process context is too slow.

Well, I'm hoping to prove the opposite. I'm working on some stuff that I
plan to present at Linux Plumbers. I've been too distracted by other
things, but hopefully I'll have some good numbers to present by then.

-- Steve


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