linux-kernel - Re: 2.6.27rc1 cannot boot more than 8CPUs

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Date:	Mon, 11 Aug 2008 22:08:08 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Yinghai Lu <yhlu.kernel@...il.com>
Cc:	Jeff Chua <jeff.chua.linux@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Miller <davem@...emloft.net>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Li Zefan <lizf@...fujitsu.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: 2.6.27rc1 cannot boot more than 8CPUs


* Yinghai Lu <yhlu.kernel@...il.com> wrote:

> On Mon, Aug 11, 2008 at 12:59 PM, Ingo Molnar <mingo@...e.hu> wrote:
> >
> > * Jeff Chua <jeff.chua.linux@...il.com> wrote:
> >
> >> On Wed, Aug 6, 2008 at 2:42 PM, Jeff Chua <jeff.chua.linux@...il.com> wrote:
> >> > On Wed, Aug 6, 2008 at 2:01 PM, Linus Torvalds
> >> > <torvalds@...ux-foundation.org> wrote:
> >> >> One trivial thing to try would be to just bisect it. I assume 2.6.26 is
> >> > Bisecting now.
> >>
> >> Thanks to all the great helpful suggestions from everyone, and this
> >> turns out that I just need to enable the following switches, so I
> >> didn't bisect further, and since it's first machine that I've tried
> >> with more than 8 CPUs so I wasn't sure whether 2.6.16 has the same
> >> problem, but if you wish, I could give 2.6.16 a try.
> >>
> >> > CONFIG_X86_GENERICARCH=y
> >> > CONFIG_X86_BIGSMP=y
> >>
> >> Thank you all for the great linux kernel!
> >
> > i still consider a silent boot hang a bug we need to fix.
> >
> > bigsmp might be required to have all cpus available on your box, but the
> > kernel is still supposed to transparently fall back to less CPUs (and
> > print a warning) if it cannot do that.
> >
> in setup.c::setup_arch() after go over with madt or mptable
> 
> #if defined(CONFIG_SMP) && defined(CONFIG_X86_PC) && defined(CONFIG_X86_32)
>         if (def_to_bigsmp)
>                 printk(KERN_WARNING "More than 8 CPUs detected and "
>                         "CONFIG_X86_PC cannot handle it.\nUse "
>                         "CONFIG_X86_GENERICARCH or
> CONFIG_X86_BIGSMP.\n"); ===> here need to change "or" to "and"
> #endif
> 
> or just panic here? because screen scroll to pass it, and user will
> not notice that...

a panic is better but still quite rude and doesnt give a user a system 
under which he can build an even greater kernel [after having discovered 
the warning in the syslog] ;-)

best would be to use as many CPUs as we can support, and skip the rest 
and boot up fine. (and print the warning prominently - the user does not 
make maximum use of available physical resources)

	Ingo
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