linux-kernel - Re: [PATCH v2 1/1] doc: Add remote CPU access details and others to this_cpu

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Date:	Thu, 17 Jul 2014 19:44:10 -0400
From:	Pranith Kumar <bobby.prani@...il.com>
To:	Christoph Lameter <cl@...two.org>
CC:	rdunlap@...radead.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] doc: Add remote CPU access details and others
 to this_cpu_ops.txt

On 07/17/2014 11:26 AM, Christoph Lameter wrote:
> On Thu, 17 Jul 2014, Pranith Kumar wrote:
> 
>> I can mention that IPI is preferable. What is that you don't want mentioned? atomic_t?
> 
> Definitely not as an example. atomic_t in per cpu areas is self
> contradicting. The per cpu area is exclusively for that processor whereas
> an atomic_t is supposed to be accessed from multiple processors.
> 
>>> Remote percpu updates are extremely rare events. If the cpu is idle/asleep
>>> then usually no updates are needed because no activity is occurring on
>>> that cpu.
>>>
>>
>> Yes, -usually- that is the case. But we are talking about the extreme rare event
>> where we need to update some remote CPU`s per-cpu data without waking it up from
>> sleep/idle. How do you suggest we handle this? I don't think suggesting not to
>> use per-cpu areas because of this is a good idea, since we lose a lot of
>> performance in the most common cases.
> 
> If you modify a percpu area then that is usually done because that cpu
> needs to take some action. An IPI is fine.
> 
> Otherwise yes I would suggest not use a percpu area but a separate data
> structure for synchronization.
> 

Yes, I will add this information to the doc. Thanks!

--
Pranith
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