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Date: Thu, 01 Mar 2012 15:26:26 +0530
From: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
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To: Ingo Molnar <mingo@elte.hu>
CC: Andrew Morton <akpm@linux-foundation.org>,
        Rusty Russell <rusty@rustcorp.com.au>, Nick Piggin <npiggin@kernel.dk>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
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        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Arjan van de Ven <arjan.van.de.ven@intel.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, mc@linux.vnet.ibm.com
Subject: Re: [PATCH] cpumask: fix lg_lock/br_lock.
References: <87ehtf3lqh.fsf@rustcorp.com.au> <20120227155338.7b5110cd.akpm@linux-foundation.org> <20120228084359.GJ21106@elte.hu> <20120228132719.f375071a.akpm@linux-foundation.org> <4F4DBB26.2060907@linux.vnet.ibm.com> <20120229091732.GA11505@elte.hu> <4F4E083A.2080304@linux.vnet.ibm.com> <20120301073845.GA5350@elte.hu> <4F4F3E16.5080703@linux.vnet.ibm.com> <20120301094529.GA366@elte.hu>
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On 03/01/2012 03:15 PM, Ingo Molnar wrote:

> 
> * Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> wrote:
> 
>> We wanted to avoid doing for_each_possible_cpu() to avoid the 
>> unnecessary performance hit. [...]
> 
> That was done at the cost of making the code rather complex.
>


Sadly, yes..

 
> The thing is, *ANY* cpu-mask loop is an utter slowpath, so the 
> "performance hit" is an overstatement. There's already dozens of 
> of for_each_possible_cpu() loops in the kernel, and it's a 
> perfectly acceptable solution in such cases.
> 
> I suspect it does not matter much now as the code appears to be 
> correct, but in general we want to opt for simpler designs for 
> rare and fragile codepaths.
> 


Ok, makes sense. And now looking back at the amount of complexity
we built into this just to avoid the for_each_possible_cpu() loop,
I wonder why we went to such lengths at all! (considering what you
said above about any cpu-mask loop..)

Regards,
Srivatsa S. Bhat