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

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Date:	Tue, 15 Jul 2008 13:15:15 -0600
From:	Alex Chiang <achiang@...com>
To:	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

* 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.

Thanks.

/ac

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