linux-kernel - Re: [patch 3/4] stop_machine: implement stop_machine_from_offline

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Date:	Thu, 23 Jun 2011 11:28:45 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Suresh Siddha <suresh.b.siddha@...el.com>, mingo@...e.hu,
	tglx@...utronix.de, hpa@...or.com, trenn@...ell.com,
	prarit@...hat.com, rusty@...tcorp.com.au,
	akpm@...ux-foundation.org, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org, youquan.song@...el.com
Subject: Re: [patch 3/4] stop_machine: implement
 stop_machine_from_offline_cpu()

Hello, Peter.

On Thu, Jun 23, 2011 at 11:25:19AM +0200, Peter Zijlstra wrote:
> On Wed, 2011-06-22 at 15:20 -0700, Suresh Siddha wrote:
> > +int stop_machine_from_offline_cpu(int (*fn)(void *), void *data,
> > +                                 const struct cpumask *cpus)
> > +{
> > +       struct stop_machine_data smdata = { .fn = fn, .data = data,
> > +                                           .active_cpus = cpus };
> > +       struct cpu_stop_done done;
> > +       int ret;
> > +
> > +       /* Local CPU must be offline and CPU hotplug in progress. */
> > +       BUG_ON(cpu_online(raw_smp_processor_id()));
> > +       smdata.num_threads = num_online_cpus() + 1;     /* +1 for local */
> > +
> > +       /* No proper task established and can't sleep - busy wait for lock. */
> > +       while (!mutex_trylock(&stop_cpus_mutex))
> > +               cpu_relax();
> > +
> > +       /* Schedule work on other CPUs and execute directly for local CPU */
> > +       set_state(&smdata, STOPMACHINE_PREPARE);
> > +       cpu_stop_init_done(&done, num_online_cpus());
> > +       queue_stop_cpus_work(cpu_online_mask, stop_machine_cpu_stop, &smdata,
> > +                            &done);
> > +       ret = stop_machine_cpu_stop(&smdata);
> > +
> > +       /* Busy wait for completion. */
> > +       while (!completion_done(&done.completion))
> > +               cpu_relax();
> > +
> > +       mutex_unlock(&stop_cpus_mutex);
> > +       return ret ?: done.ret;
> > +} 
> 
> Damn thats ugly, I sure hope you're going to make those hardware folks
> pay for this :-)

Oh, I agree it's fugly.  It's trying to orchestrate stop_machine from
a CPU which doesn't have proper scheduler/task set up.  At least it's
contained in a single relatively short function.

Thanks.

-- 
tejun
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