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

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Date:	Wed, 26 Jun 2013 19:39:40 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	tglx@...utronix.de, peterz@...radead.org, tj@...nel.org,
	oleg@...hat.com, rusty@...tcorp.com.au, mingo@...nel.org,
	akpm@...ux-foundation.org, namhyung@...nel.org, walken@...gle.com,
	vincent.guittot@...aro.org, laijs@...fujitsu.com,
	rostedt@...dmis.org, wangyun@...ux.vnet.ibm.com,
	xiaoguangrong@...ux.vnet.ibm.com, sbw@....edu, fweisbec@...il.com,
	zhong@...ux.vnet.ibm.com, nikunj@...ux.vnet.ibm.com,
	linux-pm@...r.kernel.org, linux-arch@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, Dipankar Sarma <dipankar@...ibm.com>
Subject: Re: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent
 CPU offline

On 06/26/2013 03:30 AM, Paul E. McKenney wrote:
> On Wed, Jun 26, 2013 at 01:57:55AM +0530, Srivatsa S. Bhat wrote:
>> Once stop_machine() is gone from the CPU offline path, we won't be able
>> to depend on disabling preemption to prevent CPUs from going offline
>> from under us.
>>
>> In RCU code, rcu_implicit_dynticks_qs() checks if a CPU is offline,
>> while being protected by a spinlock. Use the get/put_online_cpus_atomic()
>> APIs to prevent CPUs from going offline, while invoking from atomic context.
> 
> I am not completely sure that this is needed.  Here is a (quite possibly
> flawed) argument for its not being needed:
> 
> o	rcu_gp_init() holds off CPU-hotplug operations during
> 	grace-period initialization.  Therefore, RCU will avoid
> 	looking for quiescent states from CPUs that were offline
> 	(and thus in an extended quiescent state) at the beginning
> 	of the grace period.
> 
> o	If force_qs_rnp() is looking for a quiescent state from
> 	a given CPU, and if it senses that CPU as being offline,
> 	then even without synchronization we know that the CPU
> 	was offline some time during the current grace period.
> 
> 	After all, it was online at the beginning of the grace
> 	period (otherwise, we would not be looking at it at all),
> 	and our later sampling of its state must have therefore
> 	happened after the start of the grace period.  Given that
> 	the grace period has not yet ended, it also has to happened
> 	before the end of the grace period.
> 
> o	Therefore, we should be able to sample the offline state
> 	without synchronization.
>

Thanks a lot for explaining the synchronization design in detail, Paul!
I agree that get/put_online_cpus_atomic() is not necessary here.

Regarding the debug checks under CONFIG_DEBUG_HOTPLUG_CPU, to avoid
false-positives, I'm thinking of introducing a few _nocheck() variants,
on a case-by-case basis, like cpu_is_offline_nocheck() (useful here in RCU)
and for_each_online_cpu_nocheck() (useful in percpu-counter code, as
pointed out by Tejun Heo). These fine synchronization details are kinda
hard to encapsulate in that debug logic, so we can use the _nocheck()
variants here to avoid getting splats when running with DEBUG_HOTPLUG_CPU
enabled.

Regards,
Srivatsa S. Bhat

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