linux-kernel - Re: [patch 1/2] stop_machine: enable __stop_machine() to be called from the cpu online path

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Message-Id: <1307473957.21608.4480.camel@sbsiddha-MOBL3.sc.intel.com>
Date:	Tue, 07 Jun 2011 12:12:37 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"trenn@...ell.com" <trenn@...ell.com>,
	"prarit@...hat.com" <prarit@...hat.com>,
	"tj@...nel.org" <tj@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Song, Youquan" <youquan.song@...el.com>,
	"stable@...nel.org" <stable@...nel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [patch 1/2] stop_machine: enable __stop_machine() to be called
 from the cpu online path

On Tue, 2011-06-07 at 11:42 -0700, H. Peter Anvin wrote:
> On 06/07/2011 11:38 AM, Suresh Siddha wrote:
> > 
> > This is kind of a false positive. We are just checking if the calling
> > cpu is online/offline (and there is no possible process migration
> > between an online and offline cpu).
> > 
> 
> Is the state not available in a percpu variable of some sort?  If not,
> shouldn't it be rather than indirecting via a CPU number?

This is what I first thought about but the current percpu cpu_state is
arch specific and not all architectures define it.

Thinking a bit more, I should be able to use the stop machine's percpu
thread state and use it here. Something like the appended. I am testing
and will send the modified patches shortly.

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index c78b0c2..97a1770 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -153,7 +153,7 @@ static DEFINE_PER_CPU(struct cpu_stop_work, stop_cpus_work);
  */
 int __stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg)
 {
-	int online = cpu_online(smp_processor_id());
+	int online = percpu_read(cpu_stopper.enabled);
 	int include_this_offline = 0;
 	struct cpu_stop_work *work;
 	struct cpu_stop_done done;
@@ -513,13 +513,13 @@ int __stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
 					    .active_cpus = cpus };
 
 	/* Include the calling cpu that might not be online yet. */
-	if (!cpu_online(smp_processor_id()))
+	if (!percpu_read(cpu_stopper.enabled))
 		smdata.num_threads++;
 
 	/* Set the initial state and stop all online cpus. */
 	set_state(&smdata, STOPMACHINE_PREPARE);
 
-	if (cpu_online(smp_processor_id()))
+	if (percpu_read(cpu_stopper.enabled))
 		return stop_cpus(cpu_online_mask, stop_machine_cpu_stop,
 				 &smdata);
 	else


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