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

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Date:	Tue, 28 Oct 2008 08:17:38 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Manfred Spraul <manfred@...orfullife.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

On Tue, Oct 28, 2008 at 06:30:24AM +0100, Manfred Spraul wrote:
> Paul E. McKenney wrote:
>> On Mon, Oct 27, 2008 at 08:48:00PM +0100, Manfred Spraul wrote:
>>   
>>> Paul E. McKenney wrote:
>>>     
>>>> Agreed.  Perhaps a good change to make while introducing stall detection
>>>> to preemptable RCU -- there would then be three examples, which should
>>>> allow good generalization.
>>>>         
>>> Two implementations. IMHO the current rcu-classic code should be dropped 
>>> immediately when you add rcu-tree:
>>> rcu-classic is buggy, as far as I can see long-running interrupts on nohz 
>>> cpus are not handled correctly. I don't think it makes sense to keep it 
>>> in the kernel in parallel to rcu-tree.
>>>
>>> I would propose that rcu-tree replaces rcu-classic.
>>> I'll continue to update rcu-state, I think that it will achieve lower 
>>> latency than rcu-tree [average/max time between call_rcu() and 
>>> destruction callback] and it doesn't have the irq disabled loop to find 
>>> the missing cpus.
>>> If I find decent benchmarks where I can quantify the advantages, then 
>>> I'll propose to merge rcu-state as a third implementation in addition to 
>>> rcu-tree and rcu-preempt.
>>>
>>> Paul: What do you think?
>>
>> In keeping with my reputation as a "conservative programmer", I would
>> suggest that rcuclassic.c remain for a year or so.  Distros branching
>> off during this time should continue making rcuclassic.c be the default.
>> Other uses should have rcutree.c as the default.  At the end of the year,
>> we remove rcuclassic.c.
>>
>> All that said, one attractive aspect of your suggestion is immediately
>> removing rcuclassic.c would eliminate the need to do further work on it.  
>> ;-)
>>   
> 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.

> I would create a separate patch that removes rcuclassic.c. distros that 
> want to keep rcuclassic could just revert that change.

That does make a lot of sense.  At least it would make my life simple.  ;-)

							Thanx, Paul

> --
>    Manfred
>> Your benchmarking proposal for rcu-state makes sense to me.
>>
>> One other possible place for techniques from rcu-state may be in making
>> preemptable RCU scale.  This may take some time, as other parts of
>> the RT kernel have their limitations, but sooner or later people are
>> going to expect real-time response from even the largest machines.
>> In addition, preemptable RCU has a number of shorter-term issues:
>>
>> 1.	RCU-boosting mechanism.  (I need to combine the best of
>> 	Steve's and my mechanisms.  The treercu.c effort has been
>> 	sort of a warm-up exercise for RCU-boosting.)
>>
>> 2.	Reducing the latency contribution of the preemptable RCU
>> 	state machine (but note that moving this state machine out
>> 	of the scheduling-clock irq handler means more stuff to boost).
>>
>> 3.	Porting the simpler dynticks interface from rcutree to
>> 	preemptable RCU.
>>
>> 4.	Making the preemptable RCU tracing code use seqfile.
>>
>> Hmmm...  Maybe it is (past) time for me to publish an RCU to-do list?
>>
>> 							Thanx, Paul
>>   
>
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