linux-kernel - Re: [PATCH -next] x86/apic: Reduce print level of CPU limit announcement

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Date:   Tue, 26 Mar 2019 16:12:27 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86 <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        linux-pm <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] x86/apic: Reduce print level of CPU limit announcement

On Tue, Mar 26, 2019 at 3:41 PM Leon Romanovsky <leon@...nel.org> wrote:
>
> On Tue, Mar 26, 2019 at 01:29:54PM +0100, Rafael J. Wysocki wrote:
> > On Tue, Mar 26, 2019 at 1:02 PM Leon Romanovsky <leon@...nel.org> wrote:
> > >
> > > From: Leon Romanovsky <leonro@...lanox.com>
> > >
> > > Kernel is booted with less possible CPUs (possible_cpus kernel boot
> > > option) than available CPUs will have prints like this:
> > >
> > > [    1.131039] APIC: NR_CPUS/possible_cpus limit of 8 reached. Processor 55/0x1f ignored.
> > > [    1.132228] ACPI: Unable to map lapic to logical cpu number
> > >
> > > Those warnings are printed for every not-enabled CPU and on the systems
> > > with large number of such CPUs, we see a lot of those prints for default
> > > print level.
> > >
> > > Simple conversion of those prints to be in debug level removes them
> > > while leaving the option to debug system.
> >
> > But generally dynamic debug must be enabled in order for pr_debug()
> > prints to be visible which is kind of cumbersome to do via the command
> > line.
>
> It is doable and documented pretty well, which is uncommon :)
> https://www.kernel.org/doc/html/latest/admin-guide/dynamic-debug-howto.html#debug-messages-during-boot-process

I know.

That's what I mean by "kind of cumbersome", because you need to know
which debug messages to enable upfront.

> >
> > > Signed-off-by: Leon Romanovsky <leonro@...lanox.com>
> > > ---
> > >  arch/x86/kernel/acpi/boot.c | 2 +-
> > >  arch/x86/kernel/apic/apic.c | 6 +++---
> > >  2 files changed, 4 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
> > > index 8dcbf6890714..3ef8ab89c02d 100644
> > > --- a/arch/x86/kernel/acpi/boot.c
> > > +++ b/arch/x86/kernel/acpi/boot.c
> > > @@ -770,7 +770,7 @@ int acpi_map_cpu(acpi_handle handle, phys_cpuid_t physid, u32 acpi_id,
> > >
> > >         cpu = acpi_register_lapic(physid, acpi_id, ACPI_MADT_ENABLED);
> > >         if (cpu < 0) {
> > > -               pr_info(PREFIX "Unable to map lapic to logical cpu number\n");
> > > +               pr_debug(PREFIX "Unable to map lapic to logical cpu number\n");
> >
> > And this one is printed sometimes when something really goes wrong
> > which may be really hard to debug otherwise, so there is value in the
> > info level here.
> >
> > Would it be possible to avoid printing it just in some cases?
>
> This can do the trick:
>
> diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
> index 3ef8ab89c02d..00212b3991e0 100644
> --- a/arch/x86/kernel/acpi/boot.c
> +++ b/arch/x86/kernel/acpi/boot.c
> @@ -770,7 +770,10 @@ int acpi_map_cpu(acpi_handle handle, phys_cpuid_t physid, u32 acpi_id,
>
>         cpu = acpi_register_lapic(physid, acpi_id, ACPI_MADT_ENABLED);
>         if (cpu < 0) {
> -               pr_debug(PREFIX "Unable to map lapic to logical cpu number\n");
> +               if (cpu == -ENOENT)
> +                       pr_debug(PREFIX "Unable to map lapic to logical cpu number\n");

I don't think it is necessary to print this in the -ENOENT case, as
there is a message for that case that will be printed anyway.

> +               else
> +                       pr_info(PREFIX "Unable to map lapic to logical cpu number\n");
>                 return cpu;
>         }
>