Date: Thu, 1 Oct 2009 20:48:24 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Linus Torvalds <torvalds@linux-foundation.org>,
       Avi Kivity <avi@redhat.com>, Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Tejun Heo <tj@kernel.org>, jeff@garzik.org, linux-kernel@vger.kernel.org,
       akpm@linux-foundation.org, jens.axboe@oracle.com, rusty@rustcorp.com.au,
       cl@linux-foundation.org, dhowells@redhat.com, arjan@linux.intel.com
Subject: Re: [PATCH 03/19] scheduler: implement workqueue scheduler class
Message-ID: <20091001184824.GA21357@elte.hu>
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Thu, 1 Oct 2009, Tejun Heo wrote:
> >
> > Implement workqueue scheduler class.  Workqueue sched_class inherits 
> > fair sched_class and behaves exactly the same as sched_class except 
> > that it has two callback functions which get called when a task is 
> > put to sleep and wakes up and doesn't allow switching to different 
> > scheduler class.
> 
> So this looks odd to me.
> 
> I agree completely with the callback functions, but what I don't agree 
> with is that this is somehow workqueue-related. I bet that others 
> could use this, and in fact, I suspect that it should not be tied to 
> the scheduler class at all, but to the _thread_.
> 
> Just as an example, I could imagine that we would do lock_kernel() 
> releasing the kernel lock on scheduling (and re-taking it on wakeup) 
> as a callback. Or async IO handling - both done independently of any 
> "workqueue" logic.
> 
> So tying this to the scheduler class seems a bit odd. But maybe I'm 
> missing something?

We could do what Avi suggested: not use scheduler classes at all for 
this (that brings in other limitations like lack of p->policy freedom), 
but use the existing preempt-notifications callbacks.

They are per task - we would simply make preempt notifiers 
unconditional, i.e. remove CONFIG_PREEMPT_NOTIFIERS and make it all 
unconditional scheduler logic.

( Sidenote: we could even unify that logic with the perf event 
  sched-in/sched-out logic which is per task in a similar fashion and 
  which can have callback properties as well. That would mean a single,
  well-defined callback facility for scheduler events. )

	Ingo
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