linux-kernel - Re: [PATCH] cpu/hotplug: Cache number of online CPUs

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Message-ID: <alpine.DEB.2.21.1907042302570.1802@nanos.tec.linutronix.de>
Date:   Thu, 4 Jul 2019 23:10:10 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:     linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>,
        Nadav Amit <namit@...are.com>
Subject: Re: [PATCH] cpu/hotplug: Cache number of online CPUs

On Thu, 4 Jul 2019, Mathieu Desnoyers wrote:

> ----- On Jul 4, 2019, at 4:42 PM, Thomas Gleixner tglx@...utronix.de 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 <tglx@...utronix.de>
> > ---
> > 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