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

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Date:	Fri, 14 Sep 2007 14:38:16 +0200
From:	Pavel Machek <pavel@....cz>
To:	Thomas Gleixner <tglx@...utronix.de>
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

Hi!

> > > What was the last known to work version ?
> > 
> > I'm afraid I only turned on HIGH_RES_TIMERS in 2.6.23-rc1
> > timeframe... so I'm not sure if it ever worked for me.
> > 
> > I can confirm it is working in 2.6.23-rc5 with highres disabled, and
> > broken with highres enabled. NOHZ turns "waits for keypress during
> > unplug/replug" into "just plain hangs".
> 
> Ok, I can reproduce it and I tracked down what happens:
> 
> When the CPU goes offline, the clock event source for this CPU (lapic)
> is removed from the clock events framework. This also clears the
> information that the CPU is using C-States which stop the local APIC
> timer.
> 
> Now you put the CPU online again and the local APIC timer is used, but
> the C-State information is not evaluated again in ACPI. This means that
> the clock events code does not know that the APIC might stop. In the
> worst case this will happen and make the CPU wait for timer interrupts
> forever.
> 
> The problem only appears when you are on battery (c3/c4 available) or on
> those broken machines, where C2 is in reality C3 (e.g. akpm's VAIO)
> 
> 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?
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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