On Thu, 4 Jul 2019, Mathieu Desnoyers wrote:
> ----- 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 ?
Errm, that's the update function. And this is better called from a hotplug
lock held region and not from some random crappy code.
> 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.
What for?
num_online_cpus() is racy today vs. CPU hotplug operations as
long as you don't hold the hotplug lock.
Thanks,
tglx
