Messages in this thread |  | | Date | Thu, 26 Jan 2012 15:22:57 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] Avoid mask based num_possible_cpus and num_online_cpus -v4 |
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On Tue, 24 Jan 2012 15:25:41 -0800 Venkatesh Pallipadi <venki@google.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.
> > ... > > --- 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.
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.
> #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).
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