linux-kernel - Re: cpu hotplug support broken in 2.6.23-rc3

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Date:	Fri, 14 Sep 2007 14:50:57 +0200
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Pavel Machek <pavel@....cz>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Jeff Chua <jeff.chua.linux@...il.com>, rusty@...tycorp.com.au,
	vatsa@...ibm.com, zwane@....linux.org.uk,
	kernel list <linux-kernel@...r.kernel.org>,
	Len Brown <lenb@...nel.org>
Subject: Re: cpu hotplug support broken in 2.6.23-rc3

Pavel,

On Fri, 2007-09-14 at 14:38 +0200, Pavel Machek wrote:
> > I have an yet untested fix, which preserves the broadcast state across
> > the offline state, but Len is looking into it as well, whether we can
> > just reevaluate the power states (and the broadcast flags) when a cpu
> > becomes online again. If Len can do that easily for 2.6.23, I'd prefer
> > that.
> 
> Is there a patch you want me to test? Or does Len have anything to
> play with?

Venki sent me an initial patch, but it has issues with the notify
ordering. Find below my "cache the broadcast flags" version for testing.

Thanks,

	tglx

---
 kernel/time/tick-broadcast.c |   21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

Index: linux-2.6/kernel/time/tick-broadcast.c
===================================================================
--- linux-2.6.orig/kernel/time/tick-broadcast.c	2007-09-14 13:22:29.000000000 +0200
+++ linux-2.6/kernel/time/tick-broadcast.c	2007-09-14 13:22:29.000000000 +0200
@@ -261,10 +261,25 @@ void tick_broadcast_on_off(unsigned long
 	int cpu = get_cpu();
 
 	if (!cpu_isset(*oncpu, cpu_online_map)) {
-		printk(KERN_ERR "tick-braodcast: ignoring broadcast for "
-		       "offline CPU #%d\n", *oncpu);
-	} else {
+		unsigned long flags;
+
+		spin_lock_irqsave(&tick_broadcast_lock, flags);
+		/*
+		 * We need to cache the broadcast flag for offline
+		 * CPUs. ACPI currently does not reevaluate the
+		 * broadcast flag when a CPU goes online again. Adding
+		 * a cpu notifier to ACPI is probably the correct
+		 * solution, but it is hard to get this correct due to
+		 * notify ordering problems. So caching the flag is
+		 * the safe solution for now.
+		 */
+		if (reason == CLOCK_EVT_NOTIFY_BROADCAST_ON)
+			cpu_set(*oncpu, tick_broadcast_mask);
+		else
+			cpu_clear(*oncpu, tick_broadcast_mask);
 
+		spin_unlock_irqrestore(&tick_broadcast_lock, flags);
+	} else {
 		if (cpu == *oncpu)
 			tick_do_broadcast_on_off(&reason);
 		else


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