On 06/26/2012 03:11 PM, Daniel Lezcano wrote: > On 06/26/2012 11:29 AM, Thomas Renninger wrote: >> On Monday, June 25, 2012 06:03:42 PM Srivatsa S. Bhat wrote: >>> On 06/25/2012 07:23 PM, Thomas Renninger wrote: >>> >>>> On Monday, June 25, 2012 01:25:43 PM Srivatsa S. Bhat wrote: >>>>> >>>>> 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. >> >>> 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. >> >>> 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
