linux-kernel - Re: [git pull] scheduler fix

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Date:	Thu, 08 Jan 2009 08:50:53 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [git pull] scheduler fix

On Wed, 2009-01-07 at 15:47 -0800, Linus Torvalds wrote:
> 
> On Wed, 7 Jan 2009, Ingo Molnar wrote:
> > +		/*
> > +		 * Should not call ttwu while holding a rq->lock
> > +		 */
> > +		spin_unlock(&this_rq->lock);
> >  		if (active_balance)
> >  			wake_up_process(busiest->migration_thread);
> > +		spin_lock(&this_rq->lock);
> 
> Btw, this isn't the first time we've wanted to do a wakeup while 
> potentially locked.
> 
> Is there any way to perhaps go a "wake_up_gentle()" that doesn't need the 
> lock, and just basically does a potentially delayed wakeup by just 
> scheduling it asynchronously.
> 
> That would have solved all those nasty printk issues too. These kinds of 
> things don't need the strict "wake up NOW" behaviour - they are more of a 
> "kick the dang thing and make sure it wakes up in some timely manner".

Right -- so the printk thing was solved by polling some state from the
timer tick, we could make that into a list, but then you'd have to worry
about memory allocation for list elements failing etc.

Same story as generic smp function call, we could do this using a self
(or remote) IPI, but you'd still have the memory allocation issue -- and
would need to make self-IPI work on !SMP.

I'll ponder the issue a bit more, but I'm not directly seeing anything
(of course, if it were easy, we'd have done it ages ago ;-)

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