----- On Jul 4, 2019, at 4:42 PM, Thomas Gleixner [email protected] wrote:
> Revaluating the bitmap wheight of the online cpus bitmap in every > invocation of num_online_cpus() over and over is a pretty useless > exercise. Especially when num_online_cpus() is used in code pathes like the > IPI delivery of x86 or the membarrier code. > > Cache the number of online CPUs in the core and just return the cached > variable. > > Signed-off-by: Thomas Gleixner <[email protected]> > --- > include/linux/cpumask.h | 16 +++++++--------- > kernel/cpu.c | 16 ++++++++++++++++ > 2 files changed, 23 insertions(+), 9 deletions(-) > > --- a/include/linux/cpumask.h > +++ b/include/linux/cpumask.h > @@ -95,8 +95,13 @@ extern struct cpumask __cpu_active_mask; > #define cpu_present_mask ((const struct cpumask *)&__cpu_present_mask) > #define cpu_active_mask ((const struct cpumask *)&__cpu_active_mask) > > +extern unsigned int __num_online_cpus; [...] > + > +void set_cpu_online(unsigned int cpu, bool online) > +{ > + lockdep_assert_cpus_held(); I don't think it is required that the cpu_hotplug lock is held when reading __num_online_cpus, right ? I would have expected the increment/decrement below to be performed with a WRITE_ONCE(), and use a READ_ONCE() when reading the current value. Thanks, Mathieu > + > + if (online) { > + if (!cpumask_test_and_set_cpu(cpu, &__cpu_online_mask)) > + __num_online_cpus++; > + } else { > + if (cpumask_test_and_clear_cpu(cpu, &__cpu_online_mask)) > + __num_online_cpus--; > + } > +} > + > /* > * Activate the first processor. > */ -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
