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From: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
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>,
	Andi Kleen <ak@linux.intel.com>,
	linux-fsdevel@vger.kernel.org,
	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.
Date: Thu, 01 Mar 2012 15:26:26 +0530	[thread overview]
Message-ID: <4F4F47CA.2060900@linux.vnet.ibm.com> (raw)
In-Reply-To: <20120301094529.GA366@elte.hu>

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


  reply	other threads:[~2012-03-01  9:56 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-02-27 23:22 [PATCH] cpumask: fix lg_lock/br_lock Rusty Russell
2012-02-27 23:53 ` Andrew Morton
2012-02-27 23:53   ` Andrew Morton
2012-02-28  8:43   ` Ingo Molnar
2012-02-28 11:25     ` Andi Kleen
2012-02-28 12:51       ` Ingo Molnar
2012-02-28 21:27     ` Andrew Morton
2012-02-29  5:44       ` Srivatsa S. Bhat
2012-02-29  9:17         ` Ingo Molnar
2012-02-29 11:12           ` Srivatsa S. Bhat
2012-03-01  7:38             ` Ingo Molnar
2012-03-01  9:15               ` Srivatsa S. Bhat
2012-03-01  9:45                 ` Ingo Molnar
2012-03-01  9:56                   ` Srivatsa S. Bhat [this message]
2012-03-01  8:12             ` Srivatsa S. Bhat
2012-03-01  8:24               ` Srivatsa S. Bhat
2012-03-01  8:12               ` Srivatsa S. Bhat
2012-03-01  8:15               ` [PATCH 1/3] CPU hotplug: Fix issues with callback registration Srivatsa S. Bhat
2012-03-01  8:27                 ` Srivatsa S. Bhat
2012-03-01  8:15                 ` Srivatsa S. Bhat
2012-03-01  8:16               ` [PATCH 2/3] CPU hotplug, arch/powerpc: Fix CPU hotplug " Srivatsa S. Bhat
2012-03-01  8:28                 ` Srivatsa S. Bhat
2012-03-01  8:16                 ` Srivatsa S. Bhat
2012-03-01  8:18               ` [PATCH 3/3] CPU hotplug, arch/sparc: " Srivatsa S. Bhat
2012-03-01  8:30                 ` Srivatsa S. Bhat
2012-03-01  8:18                 ` Srivatsa S. Bhat
2012-02-29  8:29       ` [PATCH] cpumask: fix lg_lock/br_lock Ingo Molnar
2012-02-29  8:58         ` Peter Zijlstra
2012-02-29  9:32           ` Ingo Molnar
2012-02-28 11:24   ` Andi Kleen
2012-03-05  7:02     ` Rusty Russell
2012-03-05  7:03     ` [PATCH 1/3] lglock: remove online variants of lock Rusty Russell
2012-04-20 11:12       ` Nick Piggin
2012-03-05  7:04     ` [PATCH 2/3] brlocks/lglocks: API cleanups Rusty Russell
2012-03-05  7:05     ` [PATCH 3/3] brlocks/lglocks: turn into functions Rusty Russell
2012-04-20 11:21       ` Nick Piggin
2012-05-07  3:39         ` Rusty Russell
2012-05-07  5:46           ` Al Viro
2012-05-08  3:59             ` [PATCH 1/3] lglock: remove online variants of lock Rusty Russell
2012-05-08  4:50               ` Al Viro
2012-05-08  6:12                 ` Rusty Russell
2012-05-08  4:02             ` [PATCH 2/3] brlocks/lglocks: API cleanups Rusty Russell
2012-05-08  4:02             ` [PATCH 3/3] brlocks/lglocks: turn into functions Rusty Russell
2012-05-09  7:35           ` Nick Piggin
2012-05-09  7:35             ` Nick Piggin

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