linux-kernel - Re: [RFC PATCH v2 02/10] CPU hotplug: Provide APIs for "full" atomic readers to prevent CPU offline

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Date:	Thu, 06 Dec 2012 10:01:23 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Tejun Heo <tj@...nel.org>
CC:	tglx@...utronix.de, peterz@...radead.org,
	paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
	vincent.guittot@...aro.org, oleg@...hat.com, sbw@....edu,
	amit.kucheria@...aro.org, rostedt@...dmis.org, rjw@...k.pl,
	wangyun@...ux.vnet.ibm.com, xiaoguangrong@...ux.vnet.ibm.com,
	nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 02/10] CPU hotplug: Provide APIs for "full" atomic
 readers to prevent CPU offline

On 12/06/2012 02:27 AM, Tejun Heo wrote:
> Hello,
> 
> On Thu, Dec 06, 2012 at 02:01:35AM +0530, Srivatsa S. Bhat wrote:
>> Yes, that _sounds_ sufficient, but IMHO it won't be, in practice. The
>> *number* of call-sites that you need to convert from preempt_disable/enable
>> to get/put_online_cpus_atomic() won't be too many, however the *frequency*
>> of usage of those call-sites can potentially be very high.
> 
> I don't think that will be the case and, even if it is, doing it this
> way would make it difficult to tell.  The right thing to do is
> replacing stop_machine with finer grained percpu locking first.
> Refining it further should happen iff that isn't enough and there
> isn't an simpler solution.  So, let's please do the simple conversion
> first.
> 

Hmm, OK, that sounds like a good plan. So I'll drop the "light" and
"full" variants for now and work on providing a straight-forward
get/put_online_cpus_atomic() APIs.

Thank you!
 
Regards,
Srivatsa S. Bhat

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