linux-kernel - Re: [PATCH, RFC] v7 scalable classic RCU implementation

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Date:	Tue, 28 Oct 2008 18:21:06 +0100
From:	Manfred Spraul <manfred@...orfullife.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	linux-kernel@...r.kernel.org, cl@...ux-foundation.org,
	mingo@...e.hu, akpm@...ux-foundation.org, dipankar@...ibm.com,
	josht@...ux.vnet.ibm.com, schamp@....com, niv@...ibm.com,
	dvhltc@...ibm.com, ego@...ibm.com, laijs@...fujitsu.com,
	rostedt@...dmis.org, peterz@...radead.org, penberg@...helsinki.fi,
	andi@...stfloor.org, tglx@...utronix.de
Subject: Re: [PATCH, RFC] v7 scalable classic RCU implementation

Paul E. McKenney wrote:
>> How do you intend to handle nohz cpus?
>>     
>
> In which variant of RCU?  My current thought is to apply the rcutree.c
> version to rcupreempt.c.  If rcuclassic.c can be dropped, my thought
> would be to leave it alone -- it is unnecessarily awakening CPUs, but
> this is a non-fatal issue.
>
>   
For rcuclassic.

As far as  I can see, rcuclassic treats nohz cpus as always outside 
rcu_read_lock():
rcu_start_batch() contains
 >
 >                cpus_andnot(rcp->cpumask, cpu_online_map, nohz_cpu_mask);
 >
As soon as all cpus from rcp->cpumask reported a grace period, the 
callbacks are called.
That a bug, therefore I would drop rcuclassic as soon as rcutree is merged.

--
    Manfred

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