On 20 July 2015 at 16:06, Russell King - ARM Linux <li...@arm.linux.org.uk> wrote: > Why do we try to create the symlink for CPU devices which we haven't > "detected" yet (iow, we haven't had cpufreq_add_dev() called for)? > Surely we are guaranteed to have cpufreq_add_dev() called for every > CPU which exists in sysfs? So why not _only_ create the sysfs symlinks > when cpufreq_add_dev() is notified that a CPU subsys interface is > present? > > Sure, if the policy changes, we need to do maintanence on these symlinks, > but I see only one path down into cpufreq_add_dev_symlink(), which is: > > cpufreq_add_dev() -> cpufreq_add_dev_interface() -> > cpufreq_add_dev_symlink() > > In other words, only when we see a new CPU interface appears, not when > the policy changes. If the set of related CPUs is policy independent, > why is this information carried in the cpufreq_policy struct? > > If it is policy dependent, then I see no code which handles the effect > of a policy change where the policy->related_cpus is different. To me, > that sounds like a rather huge design hole. > > Things get worse. Reading drivers/base/cpu.c, CPU interface nodes are > only ever created - they're created for the set of _possible_ CPUs in > the system, not those which are possible and present, and there is no > unregister_cpu() API, only a register_cpu() API. So, cpufreq_remove_dev() > won't be called for CPUs which were present and are no longer present. > This appears to be a misunderstanding of CPU hotplug... > > So, cpufreq_remove_dev() will only get called when you call > subsys_interface_unregister(), not when the CPU present mask changes. > I suspect that the code in cpufreq_remove_dev() dealing with "offline" > CPUs even works... I'd recommend reading Documentation/cpu-hotplug.txt: > > | cpu_present_mask: Bitmap of CPUs currently present in the system. Not all > | of them may be online. When physical hotplug is processed by the relevant > | subsystem (e.g ACPI) can change and new bit either be added or removed > | from the map depending on the event is hot-add/hot-remove. There are > | currently no locking rules as of now. Typical usage is to init topology > | during boot, at which time hotplug is disabled. > | > | You really dont need to manipulate any of the system cpu maps. They should > | be read-only for most use. When setting up per-cpu resources almost always > | use cpu_possible_mask/for_each_possible_cpu() to iterate. > > In other words, I think your usage of cpu_present_mask in this code is > buggy in itself. > > Please rethink the design of this code - I think your original change is > mis-designed.
I wasn't able to get time in last few days for this, sorry about that.. Will try my best tomorrow to come back to this.. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
