linux-kernel - Re: [PATCH] cpumask: fix lg_lock/br

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Message-ID: <4F4F47CA.2060900@linux.vnet.ibm.com>
Date:	Thu, 01 Mar 2012 15:26:26 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Nick Piggin <npiggin@...nel.dk>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andi Kleen <ak@...ux.intel.com>, linux-fsdevel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arjan van de Ven <arjan.van.de.ven@...el.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	mc@...ux.vnet.ibm.com
Subject: Re: [PATCH] cpumask: fix lg_lock/br_lock.

On 03/01/2012 03:15 PM, Ingo Molnar wrote:

> 
> * Srivatsa S. Bhat <srivatsa.bhat@...ux.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

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