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

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Date:	Wed, 1 Feb 2012 14:01:25 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Venkatesh Pallipadi <venki@...gle.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mike Travis <travis@....com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.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, Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH] Avoid mask based num_possible_cpus and num_online_cpus
 -v5

On Tue, 31 Jan 2012 16:17:19 -0800
Venkatesh Pallipadi <venki@...gle.com> 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.

Looks good to me.

> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -604,9 +604,13 @@ EXPORT_SYMBOL(cpu_all_bits);
>  #ifdef CONFIG_INIT_ALL_POSSIBLE
>  static DECLARE_BITMAP(cpu_possible_bits, CONFIG_NR_CPUS) __read_mostly
>  	= CPU_BITS_ALL;
> +unsigned int nr_possible_cpus __read_mostly = CONFIG_NR_CPUS;
>  #else
>  static DECLARE_BITMAP(cpu_possible_bits, CONFIG_NR_CPUS) __read_mostly;
> +unsigned int nr_possible_cpus __read_mostly;
>  #endif
> +EXPORT_SYMBOL(nr_possible_cpus);

What the heck is CONFIG_INIT_ALL_POSSIBLE?

<blames Rusty>

:    1) Some archs (m32, parisc, s390) set possible_map to all 1, so we add a
:       CONFIG_INIT_ALL_POSSIBLE for this rather than break them.

Seems strange.  Do these architectures really need to initialise
cpu_possible_map at compile-time, when all the other architectures
manage to do it at runtime?

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