linux-kernel - Re: CPU hotplug: system hang on CPU hot remove during `pfmon --system-wide'

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Date:	Wed, 30 May 2007 12:42:01 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Zwane Mwaikambo <zwane@....linux.org.uk>,
	Nathan Lynch <nathanl@...tin.ibm.com>,
	Joel Schopp <jschopp@...tin.ibm.com>,
	Ashok Raj <ashok.raj@...el.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>, akpm@...ux-foundation.org,
	Dipankar <dipankar@...ibm.com>
Subject: Re: CPU hotplug: system hang on CPU hot remove during `pfmon
	--system-wide'

On Tue, 2007-05-29 at 13:56 -0700, Linus Torvalds wrote:
> 
> On Mon, 28 May 2007, Srivatsa Vaddagiri wrote:
> >
> > 	So is it settled now on what approach we are going to follow (freezer 
> > vs lock based) for cpu hotplug? I thought that Linus was not favouring freezer 
> > based approach sometime back ..
> 
> As far as I'm concerned, we should
>  - use "preempt_disable()" to protect against CPU's coming and going 
>  - use "stop_machine()" or similar that already honors preemption, and 
>    which I trust a whole lot  more than freezer.
>  - .. especially since this is already how we are supposed to be protected 
>    against CPU's going away, and we've already started doing that (for an 
>    example of this, see things like e18f3ffb9c from Andrew)

Indeed, this is how it was supposed to work.

	Note that it is possible to make stop_machine() an even larger hammer,
by scheduler mods to flush all the preempted tasks.  This would drop the
requirement for preempt_disable().

But cute as that would be, I've been waiting until someone demonstrates
an actual need...

Rusty.


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