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

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Date:   Tue, 26 Mar 2019 16:41:37 +0200
From:   Leon Romanovsky <leon@...nel.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     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 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

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

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index 8c2a487b5216..2704a2c57df1 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -2299,7 +2299,7 @@ int generic_processor_info(int apicid, int version)
 			"  Processor %d/0x%x ignored.\n", max, thiscpu, apicid);

 		disabled_cpus++;
-		return -ENODEV;
+		return -ENOENT;
 	}

 	if (num_processors >= nr_cpu_ids) {>

> >                 return cpu;
> >         }
> >
> > diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
> > index b7bcdd781651..8c2a487b5216 100644
> > --- a/arch/x86/kernel/apic/apic.c
> > +++ b/arch/x86/kernel/apic/apic.c
> > @@ -2305,9 +2305,9 @@ int generic_processor_info(int apicid, int version)
> >         if (num_processors >= nr_cpu_ids) {
> >                 int thiscpu = max + disabled_cpus;
> >
> > -               pr_warning("APIC: NR_CPUS/possible_cpus limit of %i "
> > -                          "reached. Processor %d/0x%x ignored.\n",
> > -                          max, thiscpu, apicid);
> > +               pr_debug(
> > +                       "APIC: NR_CPUS/possible_cpus limit of %i reached. Processor %d/0x%x ignored.\n",
> > +                       max, thiscpu, apicid);
>
> I completely agree with this change, though.

Thanks

>
> >
> >                 disabled_cpus++;
> >                 return -EINVAL;
> > --

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