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

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Message-ID: <1372260823.18733.263.camel@gandalf.local.home>
Date:	Wed, 26 Jun 2013 11:33:43 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	paulmck@...ux.vnet.ibm.com, David Laight <David.Laight@...LAB.COM>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.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 Wed, 2013-06-26 at 08:21 -0700, 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?

I thought the whole deal with this patchset was to remove stop_machine
from CPU hotplug. Why halt all CPUs just to remove one? stomp_machine()
is extremely intrusive for the entire system, where as one CPU making
sure all CPUs schedule isn't very intrusive at all.

I didn't think the idea of this patch set was to make CPU hotplug
faster, just less intrusive to the system.

-- Steve


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