linux-kernel - Re: [PATCH v2 15/45] rcu: Use get/put_online_cpus

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Date:	Wed, 26 Jun 2013 23:52:17 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Steven Rostedt <rostedt@...dmis.org>, paulmck@...ux.vnet.ibm.com,
	David Laight <David.Laight@...LAB.COM>, peterz@...radead.org,
	fweisbec@...il.com, linux-kernel@...r.kernel.org,
	walken@...gle.com, mingo@...nel.org, linux-arch@...r.kernel.org,
	vincent.guittot@...aro.org, xiaoguangrong@...ux.vnet.ibm.com,
	wangyun@...ux.vnet.ibm.com, nikunj@...ux.vnet.ibm.com,
	linux-pm@...r.kernel.org, rusty@...tcorp.com.au,
	namhyung@...nel.org, tglx@...utronix.de, laijs@...fujitsu.com,
	zhong@...ux.vnet.ibm.com, netdev@...r.kernel.org, oleg@...hat.com,
	sbw@....edu, akpm@...ux-foundation.org,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent
 CPU offline

On 06/26/2013 08:51 PM, Tejun Heo wrote:
> On Wed, Jun 26, 2013 at 10:51:11AM -0400, Steven Rostedt wrote:
>>> It would also increase the latency of CPU-hotunplug operations.
>>
>> Is that a big deal?
> 
> I thought that was the whole deal with this patchset - making cpu
> hotunplugs lighter and faster mostly for powersaving.  That said, just
> removing stop_machine call would be a pretty good deal and I don't
> know how meaningful reducing CPU hotunplug latency is.  Srivatsa?
> 

Keeping the hotunplug latency is important for suspend/resume, where
we take all non-boot CPUs in a loop. That's an interesting use-case
where intrusiveness doesn't matter much, but latency does. So yes,
making CPU hotplug faster is also one of the goals of this patchset.

Regards,
Srivatsa S. Bhat

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