linux-kernel - Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context

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Date:	Thu, 13 Dec 2012 01:13:55 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
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, tj@...nel.org, 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 v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline
 from atomic context

On 12/13/2012 01:06 AM, Oleg Nesterov wrote:
> On 12/12, Oleg Nesterov wrote:
>>
>> On 12/12, Srivatsa S. Bhat wrote:
>>>
>>> On 12/12/2012 10:47 PM, Oleg Nesterov wrote:
>>>>
>>>> Why it needs to be per-cpu? It can be global and __read_mostly to avoid
>>>> the false-sharing. OK, perhaps to put reader_percpu_refcnt/writer_signal
>>>> into a single cacheline...
>>>
>>> Even I realized this (that we could use a global) after posting out the
>>> series.. But do you think that it would be better to retain the per-cpu
>>> variant itself, due to the cache effects?
>>
>> I don't really know, up to you. This was the question ;)
> 
> But perhaps there is another reason to make it per-cpu...
> 
> It seems we can avoid cpu_hotplug.active_writer == current check in
> get/put.
> 
> take_cpu_down() can clear this_cpu(writer_signal) right after it takes
> hotplug_rwlock for writing. It runs with irqs and preemption disabled,
> nobody else will ever look at writer_signal on its CPU.
> 

Hmm.. And then the get/put_ on that CPU will increment/decrement the per-cpu
refcount, but we don't care.. because we only need to ensure that they don't
deadlock by taking the rwlock for read.

This sounds great!

Regards,
Srivatsa S. Bhat

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