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

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Message-ID: <20070622221529.GA27445@elte.hu>
Date:	Sat, 23 Jun 2007 00:15:29 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Daniel Walker <dwalker@...sta.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.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>, matthew.wilcox@...com,
	kuznet@....inr.ac.ru
Subject: Re: [RFC PATCH 0/6] Convert all tasklets to workqueues


* Daniel Walker <dwalker@...sta.com> wrote:

> >  - tasklets have certain fairness limitations. (they are executed in
> >    softirq context and thus preempt everything, even if there is 
> >    some potentially more important, high-priority task waiting to be 
> >    executed.)
> 
> Since -rt has been executing tasklets in process context for a long 
> time, I'm not sure this change would cause to many regressions. 
> However, it seems like implicit dependencies on "tasklets preempt 
> everything" might crop up. The other issue is if they don't "preempt 
> everything" (most of the time), what default priority do we give them 
> (all of the time)? [...]

there is no such guarantee at all (of 'instant preemption'), even with 
current, softirq-based tasklets. A tasklet might be 'stolen' by another 
CPU. It might be delayed to the next timer tick (or other softirq 
execution). Or it might be delayed into a ksoftirqd context, which 
currently runs at nice +19. So your worry of implicit execution 
dependencies is unfounded, because, if they existed, they would be bad 
(and triggerable) bugs today too.

	Ingo
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