Messages in this thread | | | Date | Wed, 18 Jan 2012 00:55:00 -0500 | From | KOSAKI Motohiro <> | Subject | Re: [PATCH] Avoid mask based num_possible_cpus and num_online_cpus |
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(1/17/12 9:07 PM), 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@google.com> > --- > include/linux/cpumask.h | 8 ++++++-- > kernel/cpu.c | 9 +++++++++ > 2 files changed, 15 insertions(+), 2 deletions(-) > > diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h > index 4f7a632..2eb04dd 100644 > --- a/include/linux/cpumask.h > +++ b/include/linux/cpumask.h > @@ -80,9 +80,13 @@ extern const struct cpumask *const cpu_online_mask; > extern const struct cpumask *const cpu_present_mask; > extern const struct cpumask *const cpu_active_mask; > > +extern int nr_online_cpus; > + > #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_cpu_ids)
nr_cpu_ids mean maximum cpu id of cpus. if cpu id are sparse, maximum id doesn't match number of cpus.
> + > #define num_present_cpus() cpumask_weight(cpu_present_mask) > #define num_active_cpus() cpumask_weight(cpu_active_mask) > #define cpu_online(cpu) cpumask_test_cpu((cpu), cpu_online_mask) > diff --git a/kernel/cpu.c b/kernel/cpu.c > index 2060c6e..eed2169 100644 > --- a/kernel/cpu.c > +++ b/kernel/cpu.c > @@ -622,6 +622,13 @@ static DECLARE_BITMAP(cpu_active_bits, CONFIG_NR_CPUS) __read_mostly; > const struct cpumask *const cpu_active_mask = to_cpumask(cpu_active_bits); > EXPORT_SYMBOL(cpu_active_mask); > > +#ifdef CONFIG_HOTPLUG_CPU > +int nr_online_cpus; > +#else > +int nr_online_cpus __read_mostly; > +#endif > +EXPORT_SYMBOL(nr_online_cpus);
You can always mark this to __read_mostly. other cpu hotplug stuff do so. Because of, I guess, cpu hotplug developers don't think hotplugging is frequently event.
> void set_cpu_possible(unsigned int cpu, bool possible) > { > if (possible) > @@ -644,6 +651,8 @@ void set_cpu_online(unsigned int cpu, bool online) > cpumask_set_cpu(cpu, to_cpumask(cpu_online_bits)); > else > cpumask_clear_cpu(cpu, to_cpumask(cpu_online_bits)); > + > + nr_online_cpus = cpumask_weight(cpu_online_mask); > }
I like this change. :)
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