linux-kernel - Re: [PATCH 0/6] sched/cpusets fixes, more changes are needed

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Date:	Thu, 25 Mar 2010 11:22:11 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Ben Blum <bblum@...gle.com>,
	Jiri Slaby <jirislaby@...il.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Miao Xie <miaox@...fujitsu.com>,
	Paul Menage <menage@...gle.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/6] sched/cpusets fixes, more changes are needed

On Wed, 2010-03-24 at 19:09 +0100, Oleg Nesterov wrote:
> On 03/24, Peter Zijlstra wrote:
> >
> > On Mon, 2010-03-15 at 10:09 +0100, Oleg Nesterov wrote:
> > >
> > > 	- do_fork() clears PF_STARTING and then calls wake_up_new_task()
> > > 	  which finally does s/WAKING/RUNNING.
> > >
> > > 	  But. Nobody can take rq->lock in between. This means a signal
> > > 	  from irq (quite possible with CLONE_THREAD) or another rt
> > > 	  thread which preempts us can lockup.
> >
> > Hmm, the signal case might indeed be a problem, however I cannot see how
> > the RT thread can be a problem because until we do wake_up_new_task()
> > the child will not be runnable and can thus not be preempted.
> 
> Indeed, but I meant the _parent_ can be preempted ;)

I still can't see how that would be a problem..

> In short. TASK_WAKING acts as a spinlock in fact. And since ttwu() can
> be called from any context, it should be irq-safe: any owner must disable
> inerrupts and preemption.

Agreed, and I think that's corrected with my patch.

> > The reason we have that TASK_WAKING stuff for fork is because
> > wake_up_new_task() needs p->cpus_allowed to be stable
> 
> Sure! But it is very easy to change wake_up_new_task() to set TASK_WAKING
> like ttwu() does. Of course, this needs raw_spin_lock_irq(rq->lock) for
> a moment, but afaics that is all?

My patch does that.

> > So the below patch makes select_task_rq_fair unlock the rq when needed,
> > and then puts all ->select_task_rq() calls under rq->lock. This should
> > allow us to remove the TASK_WAKING thing from fork which in turn allows
> > us to remove the PF_STARTING check in task_is_waking.
> >
> > How does that look?
> 
> I'll try to read this patch tomorrow. But could you please consider
> the suggestion above?

I think I do all those :-)

I was still looking at removing the TASK_WAKING check from
task_rq_lock() since now we do set_task_cpu() under rq->lock again so it
should be good again.

Hmm, except for sched_fork() that still does set_task_cpu() without
holding rq->lock, but that is before the child gets exposed so there
should not be any concurrency.

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