linux-kernel - Re: Bulk CPU Hotplug (Was Re: [PATCH] Do not force shutdown/reboot to boot cpu.)

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Message-ID: <20130415160908.GA17950@redhat.com>
Date:	Mon, 15 Apr 2013 18:09:08 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Robin Holt <holt@....com>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Russ Anderson <rja@....com>, Shawn Guo <shawn.guo@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Tejun Heo <tj@...nel.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Michel Lespinasse <walken@...gle.com>,
	"rusty@...tcorp.com.au" <rusty@...tcorp.com.au>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: Bulk CPU Hotplug (Was Re: [PATCH] Do not force shutdown/reboot
	to boot cpu.)

On 04/15, Robin Holt wrote:
>
> On Sat, Apr 13, 2013 at 06:30:22PM +0200, Oleg Nesterov wrote:
> > On 04/12, Robin Holt wrote:
> > >
> > > +void migrate_to_boot_cpu(void)
> > > +{
> > > +	/* The boot cpu is always logical cpu 0 */
> > > +	int reboot_cpu_id = 0;
> > > +
> > > +	/* Make certain the cpu I'm about to reboot on is online */
> > > +	if (!cpu_online(reboot_cpu_id))
> > > +		reboot_cpu_id = smp_processor_id();
> > > +
> > > +	/* Make certain I only run on the appropriate processor */
> > > +	set_cpus_allowed_ptr(current, cpumask_of(reboot_cpu_id));
> >
> > This is only theoretical, but perhaps it makes sense to set
> > PF_THREAD_BOUND before set_cpus_allowed_ptr() ? To prevent the
> > race with another thread doing sched_setaffinity().
>
> I don't quite understand this comment.  We are migrating our own thread.
> How does setting PF_THREAD_BOUND have any effect?

Suppose that another thread does, say, sys_sched_setaffinity(our_pid)
right after set_cpus_allowed_ptr(). If we set PF_THREAD_BOUND the
next set_cpus_allowed() will fail.

Oleg.

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