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

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Message-ID: <51CB32E0.3040106@linux.vnet.ibm.com>
Date:	Wed, 26 Jun 2013 23:58:48 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Steven Rostedt <rostedt@...dmis.org>, peterz@...radead.org,
	fweisbec@...il.com, oleg@...hat.com, 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, paulmck@...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, linux-kernel@...r.kernel.org, sbw@....edu,
	David Laight <David.Laight@...LAB.COM>,
	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 10:59 PM, Tejun Heo wrote:
> Hello,
> 
> On Wed, Jun 26, 2013 at 11:33:43AM -0400, Steven Rostedt wrote:
>> 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.
> 
> Yeap, removal of stop_machine is a great improvement in itself.

Absolutely. To make hotplug less intrusive on the system.

>  ISTR
> mentions of hot-unplug latency but I could be mistaken.  Srivatsa, can
> you please chime in on that?
> 

Yes, we were discussing hot-unplug latency for use-cases such as
suspend/resume. We didn't want to make those operations slower in the
process of removing stop_machine() from hotplug.

Regards,
Srivatsa S. Bhat

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