linux-kernel - Re: [PATCH 01/14] Introduce cpu_enabled

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Message-ID: <20080718214454.GD25816@flint.arm.linux.org.uk>
Date:	Fri, 18 Jul 2008 22:44:55 +0100
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	Alex Chiang <achiang@...com>, Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-acpi@...r.kernel.org
Subject: Re: [PATCH 01/14] Introduce cpu_enabled_map and friends

On Tue, Jul 15, 2008 at 01:15:15PM -0600, Alex Chiang wrote:
> * Russell King <rmk+lkml@....linux.org.uk>:
> > On Tue, Jul 15, 2008 at 12:16:32PM -0600, Matthew Wilcox wrote:
> > > On Tue, Jul 15, 2008 at 11:57:40AM -0600, Alex Chiang wrote:
> > > > My thought was that big SMP systems like ia64, possibly sparc and
> > > > ppc, and increasingly, x86, might find something like this
> > > > useful, as systems get larger and larger, and vendors are going
> > > > to want to do RAS-ish features, like the ability to keep CPUs in
> > > > firmware across reboots until told otherwise by the sysadmin.
> > > > 
> > > > Right now, a 'present' CPU strongly implies 'online' as well,
> > > > since we're calling cpu_up() for all 'present' CPUs in
> > > > smp_init(). But this hurts if:
> > > > 
> > > > 	- you don't actually want to bring up all 'present' CPUs
> > > > 	- you still want to interact with these weirdo zombie
> > > > 	  CPUs that are 'present' but not 'online'
> > > 
> > > Have you considered simply failing __cpu_up() for CPUs that are
> > > deconfigured by firmware?
> > 
> > But what if you want to have a system boot with, say, 4 CPUs and
> > then decide at run time to bring up another 4 CPUs when required?
> > 
> > How about having smp_init() call into arch code to query whether
> > it should bring up a not-already-online CPU?  Architectures that
> > want to do something special can then make the decision there and
> > everyone else can define the test completely away.
> 
> So this is exactly what I'm doing. The ia64 patch has this hunk:
> 
> @@ -820,6 +824,9 @@ __cpu_up (unsigned int cpu)
>         if (cpu_isset(cpu, cpu_callin_map))
>                 return -EINVAL;
> 
> +       if (!cpu_isset(cpu, cpu_enabled_map))
> +               return -EINVAL;
> +
>         per_cpu(cpu_state, cpu) = CPU_UP_PREPARE;
>         /* Processor goes to start_secondary(), sets online flag */
>         ret = do_boot_cpu(sapicid, cpu);
> 
> That was the easiest, most-straightforward solution I could think
> of. If you have an idea for a version with lower taxes (doesn't
> touch all the archs or can be #define'd out), I'm happy to hear
> it.

I think I did make a suggestion in the bit you quote from me above.

Let me be more explicit:

static void __init smp_init(void)
{
        unsigned int cpu;

        /* FIXME: This should be done in userspace --RR */
        for_each_present_cpu(cpu) {
                if (num_online_cpus() >= setup_max_cpus)
                        break;
-		if (!cpu_online(cpu))
+		if (smp_cpu_enabled(cpu) && !cpu_online(cpu))
                        cpu_up(cpu);
        }

        /* Any cleanup work */
        printk(KERN_INFO "Brought up %ld CPUs\n", (long)num_online_cpus());
        smp_cpus_done(setup_max_cpus);
}

and have architectures provide 'smp_cpu_enabled(cpu)' which can either
be a function, inline function or a macro (and therefore possible to be
completely eliminated.)

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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