linux-kernel - Re: [PATCH 03/19] scheduler: implement workqueue scheduler class

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Date:	Thu, 1 Oct 2009 13:03:36 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Avi Kivity <avi@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Tejun Heo <tj@...nel.org>, jeff@...zik.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	jens.axboe@...cle.com, rusty@...tcorp.com.au,
	cl@...ux-foundation.org, dhowells@...hat.com, arjan@...ux.intel.com
Subject: Re: [PATCH 03/19] scheduler: implement workqueue scheduler class

On Thu, 1 Oct 2009, Ingo Molnar wrote:
> 
> Yeah. That scheme was hobbled by signal semantics: it looked hard to do 
> the 'flip a reserve thread with a blocked thread' trick in the scheduler 
> while still keeping all the signal details in place.

I think we should look at David Wheeler's advice: "Any problem in computer 
science can be solved with another level of indirection".

In particular, my favourite solution to this is to split "struct 
thread_struct" into a new part, which would be "CPU state". 

In other words, all threadlets would share one single "struct 
thread_struct" (and thus local signal state), but they would then each 
have a "struct cpu_state" associated with them, which includes the kernel 
stack. And that cpu_state thing would not be preempted, it would be 
something like a round-robin cooperative scheduling that is only invoced 
when a threadlet goes to sleep.

			Linus
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