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

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Date:	Wed, 12 Dec 2012 20:36:46 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.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 12/12, 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.

Oleg.

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