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

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Message-ID: <CABeCy1aPp4=v90v_wAsw-KWDwNs59CwwYSJ-36+4t8kMuCaK0w@mail.gmail.com>
Date:	Fri, 27 Jan 2012 15:58:34 -0800
From:	Venki Pallipadi <venki@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	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 Thu, Jan 26, 2012 at 3:22 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Tue, 24 Jan 2012 15:25:41 -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.
>>
>> ...
>>
>> +extern int nr_online_cpus;
>> +extern int nr_possible_cpus;
>> +
>>  #ifdef CONFIG_CPUMASK_OFFSTACK
>>  /* Assuming NR_CPUS is huge, a runtime limit is more efficient.  Also,
>>   * not all bits may be allocated. */
>> @@ -81,8 +84,10 @@ extern const struct cpumask *const cpu_present_mask;
>>  extern const struct cpumask *const cpu_active_mask;
>>
>>  #if NR_CPUS > 1
>> -#define num_online_cpus()    cpumask_weight(cpu_online_mask)
>> -#define num_possible_cpus()  cpumask_weight(cpu_possible_mask)
>> +
>> +#define num_online_cpus()    (nr_online_cpus)
>> +#define num_possible_cpus()  (nr_possible_cpus)
>
> This changes the return types from "unsigned int" to int.  Worse, the
> return types become dependent upon CONFIG_SMP.
>
> s/int/unsigned int/g, methinks.
>

It will be kind of confusing with nr_cpu_ids being int and online or
possible_cpus being unsigned int. But, I agree that for the sake of
existing callers, this should be unsigned int.

>>
>> ...
>>
>> --- a/kernel/cpu.c
>> +++ b/kernel/cpu.c
>> @@ -604,16 +604,23 @@ EXPORT_SYMBOL(cpu_all_bits);
>>  #ifdef CONFIG_INIT_ALL_POSSIBLE
>>  static DECLARE_BITMAP(cpu_possible_bits, CONFIG_NR_CPUS) __read_mostly
>>       = CPU_BITS_ALL;
>> +int nr_possible_cpus __read_mostly = NR_CPUS;
>
> It looks strange to see cpu_possible_bits using CONFIG_NR_CPUS whereas
> nr_possible_cpus uses NR_CPUS.  I suggest using CONFIG_NR_CPUS for
> both.
>

OK. Will change. I saw NR_CPUS generously sprinkled around in
include/linux/cpumask.h and thought that was the preferred one to use.

> Aside: that FIXME in include/linux/threads.h should get fixed - it's
> stupid.  We should fix the Kconfigs.
>
> And the legacy NR_CPUS should be banished from the kernel altogether.
>

Silly-grep shows
$ git grep NR_CPUS | grep -v CONFIG_NR_CPUS | wc -l
748
$ git grep CONFIG_NR_CPUS | wc -l
57

So, that will involve a huge patch :-).

>>  #else
>>  static DECLARE_BITMAP(cpu_possible_bits, CONFIG_NR_CPUS) __read_mostly;
>> +int nr_possible_cpus __read_mostly;
>>  #endif
>>  const struct cpumask *const cpu_possible_mask = to_cpumask(cpu_possible_bits);
>>  EXPORT_SYMBOL(cpu_possible_mask);
>>
>> +EXPORT_SYMBOL(nr_possible_cpus);
>
> It's better to place the export immediately following the nr_possible_cpus
> definition(s).

OK. Will do.

Thanks,
Venki
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