linux-kernel - Re: [PATCH] x86 boot: Wait for boot cpu to show up if nr

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Date:	Fri, 8 Jul 2011 10:48:53 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	linux kernel mailing list <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86 boot: Wait for boot cpu to show up if nr_cpus limit
 is about to hit

On Fri, Jul 8, 2011 at 10:19 AM, Vivek Goyal <vgoyal@...hat.com> wrote:
> nr_cpus allows one to specify number of possible cpus in the system.
> Current assumption seems to be that first cpu to show up is boot cpu
> and this assumption will be broken in kdump scenario where we can be
> booting on a non boot cpu with nr_cpus=1.
>
> It might happen that first cpu we parse is not the cpu we boot on and
> later we ignore boot cpu. Though code later seems to recognize this
> anomaly and forcibly sets boot cpu in physical cpu map with following
> warning.
>
> if (!physid_isset(hard_smp_processor_id(), phys_cpu_present_map)) {
>        printk(KERN_WARNING
>                "weird, boot CPU (#%d) not listed by the BIOS.\n",
>                hard_smp_processor_id());
>
>        physid_set(hard_smp_processor_id(), phys_cpu_present_map);
> }
>
> This patch waits for boot cpu to show up and starts ignoring the cpus
> once we have hit (nr_cpus - 1) number of cpus. So effectively we are
> reserving one slot out of nr_cpus for boot cpu explicitly.
>
> Signed-off-by: Vivek Goyal <vgoyal@...hat.com>

Acked-by: Yinghai Lu <yinghai@...nel.org>

> ---
>  arch/x86/kernel/apic/apic.c |   22 ++++++++++++++++++++--
>  1 file changed, 20 insertions(+), 2 deletions(-)
>
> Index: linux-2.6/arch/x86/kernel/apic/apic.c
> ===================================================================
> --- linux-2.6.orig/arch/x86/kernel/apic/apic.c  2011-07-08 10:44:48.003709709 -0400
> +++ linux-2.6/arch/x86/kernel/apic/apic.c       2011-07-08 12:02:01.272223921 -0400
> @@ -1943,10 +1943,28 @@ void disconnect_bsp_APIC(int virt_wire_s
>
>  void __cpuinit generic_processor_info(int apicid, int version)
>  {
> -       int cpu;
> +       int cpu, max = nr_cpu_ids;
> +       bool boot_cpu_detected = physid_isset(boot_cpu_physical_apicid,
> +                               phys_cpu_present_map);
> +
> +       /*
> +        * If boot cpu has not been detected yet, then only allow upto
> +        * nr_cpu_ids - 1 processors and keep one slot free for boot cpu
> +        */
> +       if (!boot_cpu_detected && num_processors >= nr_cpu_ids - 1 &&
> +           apicid != boot_cpu_physical_apicid) {
> +               int thiscpu = max + disabled_cpus - 1;
> +
> +               pr_warning(
> +                       "ACPI: NR_CPUS/possible_cpus limit of %i almost"
> +                       " reached. Keeping one slot for boot cpu."
> +                       "  Processor %d/0x%x ignored.\n", max, thiscpu, apicid);
> +
> +               disabled_cpus++;
> +               return;
> +       }
>
>        if (num_processors >= nr_cpu_ids) {
> -               int max = nr_cpu_ids;
>                int thiscpu = max + disabled_cpus;
>
>                pr_warning(
> --
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