On 06/26/2012 11:58 AM, Srivatsa S. Bhat wrote: > 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. AFAIU, this code is not called after onlining a cpu greater than maxcpus and Thomas thinks that system with cpu hotplug at runtime are not sold. The problem I see with this code is acpi and intel-idle are tied together now. I would like to break this dependency and use the notifier to handle the cpu hotplug directly in intel-idle. It is hard to test my patch as there is not such system and maxcpus is not correctly handled here. I can use your patch to test my patch but anyway ... I am just asking if that would make sense to remove this portion of code instead :) If we want to keep this code untouched, I can try my patch and maybe Thomas will agreed to test it also on a cpu-online-runtime-system if he has one. Thanks -- Daniel -- <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs Follow Linaro: <http://www.facebook.com/pages/Linaro> Facebook | <http://twitter.com/#!/linaroorg> Twitter | <http://www.linaro.org/linaro-blog/> Blog -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
