linux-kernel - Re: [PATCH v3 1/7] arm64: add support for the AMU extension v1

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Message-ID: <20200212201942.GA12970@arm.com>
Date:   Wed, 12 Feb 2020 20:19:42 +0000
From:   Ionela Voinescu <ionela.voinescu@....com>
To:     Suzuki Kuruppassery Poulose <suzuki.poulose@....com>
Cc:     catalin.marinas@....com, will@...nel.org, mark.rutland@....com,
        maz@...nel.org, sudeep.holla@....com, lukasz.luba@....com,
        valentin.schneider@....com, rjw@...ysocki.net,
        peterz@...radead.org, mingo@...hat.com, vincent.guittot@...aro.org,
        viresh.kumar@...aro.org, linux-arm-kernel@...ts.infradead.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH v3 1/7] arm64: add support for the AMU extension v1

Hi guys,

On Wednesday 12 Feb 2020 at 16:20:56 (+0000), Suzuki Kuruppassery Poulose wrote:
> > For better handling I could have a cpumask_available check before the
> > allocation just in case the capability type changes in the future, or to
> > at least not rely on assumptions based on the type of the capability.
> > 
> > The reason this is dynamic is that I wanted to avoid the memory being
> > allocated when disable_amu is true - as Valentin mentioned in a comment
> > in the meantime "the static allocation is done against NR_CPUS whereas
> > the dynamic one is done against nr_cpu_ids".
> > 
> > Would this be alright?
> > 
> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index 182e05ca3410..4cee6b147ddd 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -1222,7 +1222,11 @@ static bool has_amu(const struct arm64_cpu_capabilities *cap,
> >           * The enable function will also print a detection message.
> >           */
> > -       if (!disable_amu && !zalloc_cpumask_var(&amu_cpus, GFP_KERNEL)) {
> > +       if (disable_amu)
> > +               return false;
> > +
> > +       if (!cpumask_available(amu_cpus) &&
> > +           !zalloc_cpumask_var(&amu_cpus, GFP_KERNEL)) {
> >                  pr_err("Activity Monitors Unit (AMU): fail to allocate memory");
> >                  disable_amu = true;
> >          }

Going down the rabbit hole in regards to this section, it seems it's
actually not fine. When CONFIG_CPUMASK_OFFSTACK==y it fails to
allocate memory because zalloc_cpumask_var cannot be used before
initializing the slub allocator (mm_init) to allocate a cpumask.

The alternative alloc_bootmem_cpumask_var is an __init function that
can be used only during the initialization phase, which is not the case
for has_amu that can be called later (for hotplugged CPUs). Therefore,
dynamic allocation is not an option here.

Thanks,
Ionela.