linux-kernel - Re: [PATCH] RCU: implement rcu_read_[un]lock

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Date:	Tue, 29 Jul 2008 10:47:14 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	dipankar@...ibm.com, Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Paul E McKenney <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] RCU: implement rcu_read_[un]lock_preempt()

Peter Zijlstra wrote:
>>> If you need preempt off you need it for other reasons than RCU, so
>>> mixing it in the interface doesn't make sense to me.
>> Hmmm... the point of the interface is avoiding doing double preemption
>> operations as on common configurations rcu_read_lock() disables
>> preemption.
> 
> Should be really cheap then, because the cacheline is already hot.

Yeah, it is, so it is eventually a peripheral issue.

>>   Yes, it's for different purposes but we have two partially
>> overlapping ops and implementing combined / collapsed ops for such cases
>> is acceptable, I think.
> 
> They only overlap for !PREEMPT_RCU || !PREEMPT_RT

That part is pretty large tho.

>> Using get_cpu() or separate preempt_disable() wouldn't incur noticeable
>> performance difference as preemption is really cheap to manipulate but
>> both per-cpu and RCU are for performance optimization and I think having
>> combined ops is a good idea.
> 
> I don't as its a nightmare to sort out on -rt, where get_cpu() can be
> converted to get_cpu_locked(), and rcu_read_lock() never disables
> preemption.
> 
> If you convert it to use get_cpu() the conversion is easy, if you
> introduce this collapsed primitive we're up shit creek because it
> doesn't map.

I don't get it.  So, rcu_read_lock(); preempt_disable(); doesn't map for RT?

> Nor does it tell us why you need preempt disabled. Making it just as bad
> as open-coded preempt_disable()s.

Heh.. the code probably would have used preempt_disable() if it were not
for the combined ops, so the objection is about using preempt_disable()?

-- 
tejun
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