linux-kernel - Re: [RFC PATCH 2/4] x86: use dmi check to treat disabled cpus as hotplug cpus.

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Date:	Mon, 11 Jan 2010 19:13:32 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Yinghai Lu <yinghai@...nel.org>
cc:	Suresh Siddha <suresh.b.siddha@...el.com>,
	"ananth@...ibm.com" <ananth@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/4] x86: use dmi check to treat disabled cpus as
 hotplug cpus.



On Mon, 11 Jan 2010, Yinghai Lu wrote:
>
> some systems that have disable cpus entries because same
>   BIOS will support 2 sockets and 4 sockets and more at
>   same time, BIOS just leave some disable entries, but
>   those system do not support cpu hotplug. we don't need
>   treat disabled_cpus as hotplug cpus.
>
> so we can make nr_cpu_ids smaller and save more space
>   (pcpu data allocations), and could make some systems run
>   with logical flat instead of physical flat apic mode

.. but this one I detest.

We can't play games that depend on us always filling in some DMI table 
correctly. Things need to "just work".

So while 1/4 looks fine, 2/4 looks fundamentally unacceptable.

Is the flat APIC mode really so important?

I would suggest a few alternatives:

Truly static:

 - only use that flat apic mode when you _know_ that you absolutely will 
   never have more than 8 cpu's. Ie when CONFIG_NR_CPUS <= 8 (or, with 
   1/3, when nr_cpu_ids <= 8) and/or when <= 8 CPU's were detected, and 
   CPU hotplug is disabled entirely.

Slightly more intelligent:

 - Look at the ACPI socket count, and hey, if it says it might have more 
   sockets - whether they are really hotplug or not, don't use flat mode, 
   because we simply don't know. But do _not_ do some kind of DMI table to 
   say one way or the other.

And if it's _really_ important:

 - if flat mode is so important that you want to enable it whenever 
   possible, what about enabling/disabling it dynamically at CPU hotplug 
   time? That does sound _very_ painful, but it's still better than having 
   to maintain some list of all systems that can ever hot-plug. 

Hmm?

		Linus
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