linux-kernel - Re: [RFC PATCH 0/5] Refcount based Cpu Hotplug. V2

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Message-ID: <20071024182210.GA10884@in.ibm.com>
Date:	Wed, 24 Oct 2007 23:52:10 +0530
From:	Gautham R Shenoy <ego@...ibm.com>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Christoph Lameter <clameter@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Rusty Russel <rusty@...tcorp.com.au>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Paul E McKenney <paulmck@...ibm.com>,
	Richard Gooch <rgooch@...f.csiro.au>,
	Tigran Aivazian <tigran@...azian.fs.co.uk>,
	Shoahua Li <shaohua.li@...ux.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Nathan Lynch <ntl@...ox.com>,
	David Miller <davem@...emloft.net>, Paul Jackson <pj@....com>,
	Josh Triplett <josh@...edesktop.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Akinobu Mita <akinobu.mita@...il.com>
Subject: Re: [RFC PATCH 0/5] Refcount based Cpu Hotplug. V2

On Wed, Oct 24, 2007 at 10:17:54PM +0400, Oleg Nesterov wrote:
> On 10/24, Gautham R Shenoy wrote:
> >
> > On Wed, Oct 24, 2007 at 10:04:41AM -0700, Christoph Lameter wrote:
> > > On Wed, 24 Oct 2007, Gautham R Shenoy wrote:
> > > 
> > > > This is the version 2 of the refcount based cpu-hotplug "locking" 
> > > > implementation. 
> > > 
> > > Uggh. This introduces a global lock that has to be taken always when 
> > > scanning over cpus? 
> > 
> > Well, no! we take the global lock only while bumping up the refcount. 
> > We don't hold the lock while scanning over the cpus. And this is
> > definitely an improvement over the lock_cpu_hotplug() global mutex
> > we have now.
> 
> Just to be sure I didn't miss something... preempt_disable() still works,
> yes?

Yes it does. But it doesn't prevent onlining of new cpus. 
I am checking if __cpu_up() can also be called safely using
stop_machine_run() so that we can use preempt_disable() for safe atomic
access of the cpu_online_map.

> 
> Oleg.
> 

Thanks and Regards
gautham.
-- 
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"
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