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

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Message-ID: <20120615015331.GC12624@mtj.dyndns.org>
Date:	Fri, 15 Jun 2012 10:53:31 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <peterz@...radead.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 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.

Thanks.

-- 
tejun
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