linux-kernel - Re: [RFC PATCH 0/6] Convert all tasklets to workqueues

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Date:	Mon, 25 Jun 2007 16:07:01 -0400
From:	Kristian Høgsberg <krh@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Hellwig <hch@...radead.org>,
	john stultz <johnstul@...ibm.com>,
	Oleg Nesterov <oleg@...sign.ru>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	"David S. Miller" <davem@...emloft.net>, kuznet@....inr.ac.ru
Subject: Re: [RFC PATCH 0/6] Convert all tasklets to workqueues

On Mon, 2007-06-25 at 15:11 -0400, Steven Rostedt wrote:
> On Mon, 2007-06-25 at 14:48 -0400, Kristian Høgsberg wrote:
> ...
> > However, I don't really understand how you can discuss a wholesale
> > replacing of tasklets with workqueues, given the very different
> > execution sematics of the two mechanisms.  I would think that others
> > have used tasklets for similar purposes as I have and moving that to
> > workqueues just has to break a bunch of stuff.  I don't know the various
> > places tasklets are used as well as other people in this thread, but I
> > think deprecating them and moving code to either softirqs or workqueues
> > on a case by case basis is a better approach.  That way we also avoid
> > the gross wrappers.
> 
> The gross wrappers were a perfect way to shed light on something that is
> overused, and should most likely be replaced.
> 
> Does your system need to have these functions that are in tasklets need
> to be non-reentrant?  I wonder how many "irq critical" functions used
> tasklets just because adding a softirq requires too much (no generic
> softirq code).  A tasklet is constrained to run on one CPU at a time,
> and it is not guaranteed to run on the CPU it was scheduled on.

When I started the firewire work, I wasn't aware that tasklets were
going away, but I knew that doing too much work in the interrupt handler
was frowned upon, for good reasons.  So I was looking at softirqs vs
taslkets, and since using softirqs means you have to go add yourself to
the big bitmask, I opted for tasklets.  The comment in interrupt.h
directly recommends this.  As it stands, the firewire stack does
actaully rely on the non-reentrancy of tasklets, but that's not a
deal-breaker, I can add the necessary locking.

> Perhaps it's time to add a new functionality while removing tasklets.
> Things that are ok to bounce around CPUs (like tasklets do) can most
> likely be replaced by a work queue. But these highly critical tasks
> probably would benefit from being a softirq.
> 
> Maybe we should be looking at something like GENERIC_SOFTIRQ to run
> functions that a driver could add. But they would run only on the CPU
> that scheduled them, and do not guarantee non-reentrant as tasklets do
> today.

Sounds like this will fill the gap.  Of course, this won't reduce the
number of delayed-execution mechanisms available...

Kristian

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