linux-kernel - Re: [PATCH] Avoid mask based num_possible_cpus and num_online

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Date:	Fri, 20 Jan 2012 17:01:24 -0800
From:	Venki Pallipadi <venki@...gle.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	kosaki.motohiro@...il.com, akpm@...ux-foundation.org,
	travis@....com, srivatsa.bhat@...ux.vnet.ibm.com,
	paul.mckenney@...aro.org, rjw@...k.pl,
	paul.gortmaker@...driver.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Avoid mask based num_possible_cpus and num_online_cpus

On Thu, Jan 19, 2012 at 12:40 PM, KOSAKI Motohiro
<kosaki.motohiro@...fujitsu.com> wrote:
> On 1/19/2012 3:01 PM, Venkatesh Pallipadi wrote:
>> Does this look better? Will send separate patch to fix code
>> using num_possible_cpus() when they actually need nr_cpu_ids.
>
> Sound ok to me. but I have two requests.
>
>  - Please mesure how much time (or cycle) spented by cpumask_weight(cpu_possible_mask).

On two of the recentish x86 server CPUs, __sw_hweight64() base
mask_weight() takes ~300 cycles. popcnt instruction based weight takes
<10 cycles.

>  - After your patch, nr_possible_cpus() return different value from before.

Yes. I have tested this on x86 and haven't seen any problem.

>   Please verify this change doesn't makes any side effect.
>
>
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