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

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Message-ID: <4F218CF2.4020409@linux.vnet.ibm.com>
Date:	Thu, 26 Jan 2012 22:57:14 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Venkatesh Pallipadi <venki@...gle.com>
CC:	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mike Travis <travis@....com>,
	"Paul E. McKenney" <paul.mckenney@...aro.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Avoid mask based num_possible_cpus and num_online_cpus
 -v4

On 01/25/2012 04:55 AM, Venkatesh Pallipadi wrote:

> Kernel's notion of possible cpus (from include/linux/cpumask.h)
>  *     cpu_possible_mask- has bit 'cpu' set iff cpu is populatable
> 
>  *  The cpu_possible_mask is fixed at boot time, as the set of CPU id's
>  *  that it is possible might ever be plugged in at anytime during the
>  *  life of that system boot.
> 
>  #define num_possible_cpus()     cpumask_weight(cpu_possible_mask)
> 
> and on x86 cpumask_weight() calls hweight64 and hweight64 (on older kernels
> and systems with !X86_FEATURE_POPCNT) or a popcnt based alternative.
> 
> i.e, We needlessly go through this mask based calculation everytime
> num_possible_cpus() is called.
> 
> The problem is there with cpu_online_mask() as well, which is fixed value at
> boot time in !CONFIG_HOTPLUG_CPU case and should not change that often even
> in HOTPLUG case.
> 
> Though most of the callers of these two routines are init time (with few
> exceptions of runtime calls), it is cleaner to use variables
> and not go through this repeated mask based calculation.
> 
> Signed-off-by: Venkatesh Pallipadi <venki@...gle.com>
> ---


Looks good to me now :-)

Acked-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>

Regards,
Srivatsa S. Bhat

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