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 10:26:24 -0500 (CDT)
From:	Christoph Lameter <cl@...two.org>
To:	Pranith Kumar <bobby.prani@...il.com>
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 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.
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