linux-kernel - Re: [RFC patch 2/5] smpboot: Provide infrastructure for percpu hotplug threads

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Message-ID: <1339754286.2559.48.camel@twins>
Date:	Fri, 15 Jun 2012 11:58:06 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [RFC patch 2/5] smpboot: Provide infrastructure for percpu
 hotplug threads

On Fri, 2012-06-15 at 10:53 +0900, Tejun Heo wrote:
> On Thu, Jun 14, 2012 at 10:17:28AM +0200, Peter Zijlstra wrote:
> > On Thu, 2012-06-14 at 10:08 +0200, Peter Zijlstra wrote:
> > > On Wed, 2012-06-13 at 20:56 +0200, Thomas Gleixner wrote:
> > > > If it's just a spurious wakeup then it goes back to sleep right away
> > > > as nothing cleared the park bit. 
> > > 
> > > Your spurious wakeup will have destroyed the binding though. So you need
> > > to be careful.
> > 
> > We should probably do something like the below..
> > 
> > TJ does this wreck workqueues? Its somewhat 'creative' in that regard
> > and really wants fixing.
> > 
> > ---
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -5018,6 +5018,8 @@ void do_set_cpus_allowed(struct task_str
> >  
> >  	cpumask_copy(&p->cpus_allowed, new_mask);
> >  	p->nr_cpus_allowed = cpumask_weight(new_mask);
> > +	if (p->nr_cpus_allowed != 1)
> > +		p->flags &= ~PF_THREAD_BOUND;
> 
> The only reason wq workers use PF_THREAD_BOUND is to prevent userland
> from mucking with cpus_allowed, so the above wouldn't break anything
> in itself although userland would be able to wreck it afterwards.

Thing is, if things could get wrecked by userland moving a thread to a
different cpu, things just got wrecked by the kernel doing that very
same thing.

PF_THREAD_BOUND isn't called PF_NO_USER_AFFINITY (although it seems a
popular interpretation).


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