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SubjectRe: [PATCH] x86: Issue a warning if number of present CPUs > maxcpus and CONFIG_HOTPLUG=n
On Mon, 17 Feb 2014 10:40:07 -0300
Henrique de Moraes Holschuh <hmh@hmh.eng.br> wrote:

> On Mon, 17 Feb 2014, Petr Tesarik wrote:
> > Well, if the user passes both nr_cpus and maxcpus parameters to the
> > kernel, I think it's fair to issue two warnings. But if everyone agrees
> > that only the maxcpus warning should be printed in that case, I can
> > send a version 2 of my patch.
>
> Please remember that the market is full of motherboards with the extremely
> annoying behaviour of declaring ACPI objects for CPU cores that will never
> be available. This includes a large number of workstation and server boards
> at the very least, from at least one rather large vendor.
>
> As far as I know, we still don't have a way to realiably detect this and get
> rid of the ghost processors which will *NEVER* become online. Setting
> maxcpus or nr_cpus manually is the current way to avoid wasting runtime
> resources because of phantom cores that will never become reality.
>
> So, when you fix the bug that always supress the warnings, you will at the
> same time cause a regression on those boxes, which will now print undesired
> warnings. If the user has manually set nr_cpus or maxcpus, maybe it would
> be best to not print any warnings or alternatively to downgrade them to
> debug level?

While I appreciate your concerns, I fail to see how they are related.

First, please keep in mind that my patch does not alter the (more
common) case with CONFIG_HOTPLUG_CPU=y in any way. For the (presumably
less common) !HOTPLUG_CPU case, it adds a warning that is always issued
in the HOTPLUG_CPU case.

Second, regarding your use case, I don't think it changes anything. So,
let's say you have a board with 16 CPUs and a MADT that describes 1024
CPUs. You know that there can (physically) be at most 48 CPUs, so you
boot with nr_cpus=48:

num_processors = 16 /* online CPUs at boot */
disabled_cpus = 992 /* 1008 - 16 */

This results in:

total_cpus = 1008 /* this is purely informative, it is *NOT* used
to size anything */
possible = 48 /* clamped to nr_cpu_ids */

A warning message (with or without my patch):
1024 Processors exceeds NR_CPUS limit of 48

Informative message:
Allowing 16 CPUs, 32 hotplug CPUs

No other warning (with or without my patch).

Petr Tesarik
SUSE L3 Team 1


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