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

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Date:	Tue, 12 Jan 2010 10:13:36 -0800
From:	Yinghai Lu <yinghai@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.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 Tue, Jan 12, 2010 at 7:19 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Tue, 12 Jan 2010, Yinghai Lu wrote:
>> >
>> > 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.
>>
>> that acpi could lie, for example, some system share one BIOS between 2
>> socket/4 socket/8 sockets
>> model. and BIOS could have bunch disabled entries in MADT. or MPTABLE.
>
> That's fine. So it would mean that sometimes we'd use non-flat mode even
> if we strictly didn't need to (because we _think_ that there could be four
> sockets even though there is only one, and three are disabled). But things
> would still work without any special cases.
>
>> > 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.
>>
>> interesting, could be done.
>> init_apic_ldr is called even for physical flat on 64 bit.
>> could change apic on fly.
>
> Quite frankly, while I suggested it as an option, I really suspect it's
> too much complexity for very little real gain.
>
> Say that you have only four cores, but the kernel decided that it can't
> use logical flat APIC mode because it sees three disabled sockets and
> thinks "ok, we may end up with a total of 16 cores if those sockets are
> hotplugged". Is that such a disaster?
>
> Realistically, do we really care? Do you have performance numbers that say
> that logical flat mode is so important that we really _really_ want to use
> it, even at the cost of nasty run-time complexity with having to
> re-program the APIC setup entirely when going from 8->9 CPU's?

ok let stay with option 1.

and would like to use DMI to blacklist those systems that do need treat disabled
cpus MADT as hotplug cpus.

YH
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