Messages in this thread | | | Date | Wed, 18 Jan 2012 10:52:30 -0800 | Subject | Re: [PATCH] Avoid mask based num_possible_cpus and num_online_cpus | From | Venki Pallipadi <> |
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On Tue, Jan 17, 2012 at 9:55 PM, KOSAKI Motohiro <kosaki.motohiro@gmail.com> wrote: > (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. >
Yes. But will it be sparse in any arch? I saw some of the users of num_possible_cpus() doing things like allocating a buffer for that size and then indexing it using get_cpu(). So, I thought it would be better to use the existing nr_cpu_ids instead of inventing another variable. If indeed any arch is depending on this being sparse, we can have a new variable similar to num_possible_cpus and also audit all users of num_possible_cpus to see whether they should be using nr_cpu_ids instead.
> > >> + >> #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. >
OK. Agree.
> >> 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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