On 06/26/2012 03:11 PM, Daniel Lezcano wrote:

On 06/26/2012 11:29 AM, Thomas Renninger wrote:

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On Monday, June 25, 2012 06:03:42 PM Srivatsa S. Bhat wrote:

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On 06/25/2012 07:23 PM, Thomas Renninger wrote:

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On Monday, June 25, 2012 01:25:43 PM Srivatsa S. Bhat wrote:

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Daniel Lezcano noticed that after booting with maxcpus=X, if we online the
remaining cpus by writing: echo 1 > /sys/devices/system/cpu/cpuY/online, then
for the newly onlined cpus, the cpuidle directory is not found under
/sys/devices/system/cpu/cpuY.

Partly, the reason for this is that acpi restricts the initialization to cpus
within the maxcpus limit. (See commit 75cbfb9 "ACPI: Do not try to set up acpi
processor stuff on cores exceeding maxcpus="). The maxcpus= kernel parameter is
used to restrict the number of cpus brought up during boot. That doesn't mean
that we should hard restrict the bring up of the remaining cpus later on.

Sorry, but IMO it exaclty does mean that (adding more general lists for
further comments).

If you can online more cores than maxcpus= via sysfs, this sounds like a bug.
Not the other way around.

Compare with Documentation/kernel-parameters.txt:
        maxcpus=        [SMP] Maximum number of processors that an SMP kernel
                        should make use of.  maxcpus=n : n >= 0 limits the
                        kernel to using 'n' processors.  n=0 is a special case,
                        it is equivalent to "nosmp", which also disables
                        the IO APIC.

Chances that you run into more problems are high.

Right, I agree on that. So, IMHO, maxcpus=X doesn't mean that the kernel must and
should forbid any new cpus from coming online, but in the interest of avoiding
problems/complications in some obscure paths, I guess it makes sense to avoid
onlining new cpus beyond maxcpus.

Yep, for such reasons:
   - That nobody realizes this to be useful and makes use of it in a productive
     environment
   - If I see maxcpus=X in a bugreport's dmesg command line,
     I want to be sure that's true.
   - To enforce that things work as documented


Wow, after looking a bit into this I found (Documentation/cpu-hotplug.txt):

maxcpus=n    Restrict boot time cpus to n. Say if you have 4 cpus, using
             maxcpus=2 will only boot 2. You can choose to bring the
             other cpus later online, read FAQ's for more info.

Looks like someone already documented this (IMO broken) behavior.
I didn't find further info in the FAQs.

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In any case, I was just trying to see why the simple removal of the setup_max_cpus
check in acpi_processor_add() wasn't enough to expose the cpuidle directories under
the new cpus.. and while debugging that, I came up with this patch. I don't mind
if this doesn't get picked up.

quoted

Right, the usecase of why somebody would like to online new cpus beyond maxcpus
doesn't look all that solid anyway. So I am OK with leaving the code as it is now.

In the end this is a debug option, I expect everybody is aware of that.
Yep, let's just leave it...

In this case, let's remove the intel_idle_cpu_init stuff in
acpi_cpu_soft_notify, no ?

Why? And how would that help? The intel_idle_cpu_init() call is essential if intel_idle
driver is being used instead of acpi idle.

Regards,
Srivatsa S. Bhat