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Message-Id: <1180492921.30202.77.camel@localhost.localdomain>
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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