The information in /sys/devices/system/cpu/cpuX/topology directory is useful for userspace monitoring applications and in-tree utilities like cpupower & turbostat.
When down'ing a thread the /sys/devices/system/cpu/cpuX/topology directory is removed during the CPU_DEAD hotplug callback in the kernel. The problem with this model is that the thread's core has not been physically removed and the data in the topology directory is still valid and the core's location is now lost to userspace. This patch adds CONFIG_PERMANENT_CPU_TOPOLOGY, and is Y by default for x86, an N for all other arches. When enabled the kernel is modified so that the topology directory is added to the core cpu sysfs files so that the topology directory exists while the CPU is physically present. When disabled, the behavior of the current kernel is maintained (that is, the topology directory is removed on a soft down and added on an soft up of a thread). Adding CONFIG_PERMANENT_CPU_TOPOLOGY may require additional architecture so that the cpumask data the CPU's topology is not cleared during a CPU down. Before patch: [root@hp-z620-01 ~]# grep ^ /sys/devices/system/cpu/cpu10/topology/* /sys/devices/system/cpu/cpu10/topology/core_id:3 /sys/devices/system/cpu/cpu10/topology/core_siblings:ffff /sys/devices/system/cpu/cpu10/topology/core_siblings_list:0-15 /sys/devices/system/cpu/cpu10/topology/physical_package_id:0 /sys/devices/system/cpu/cpu10/topology/thread_siblings:0404 /sys/devices/system/cpu/cpu10/topology/thread_siblings_list:2,10 Down a cpu [root@hp-z620-01 ~]# echo 0 > /sys/devices/system/cpu/cpu10/online [root@hp-z620-01 ~]# ls /sys/devices/system/cpu/cpu10/topology ls: cannot access topology: No such file or directory After patch: [root@hp-z620-01 ~]# grep ^ /sys/devices/system/cpu/cpu10/topology/* /sys/devices/system/cpu/cpu10/topology/core_id:3 /sys/devices/system/cpu/cpu10/topology/core_siblings:ffff /sys/devices/system/cpu/cpu10/topology/core_siblings_list:0-15 /sys/devices/system/cpu/cpu10/topology/physical_package_id:0 /sys/devices/system/cpu/cpu10/topology/thread_siblings:0404 /sys/devices/system/cpu/cpu10/topology/thread_siblings_list:2,10 Down a cpu [root@hp-z620-01 ~]# echo 0 > /sys/devices/system/cpu/cpu10/online [root@hp-z620-01 ~]# grep ^ /sys/devices/system/cpu/cpu10/topology/* /sys/devices/system/cpu/cpu10/topology/core_id:3 /sys/devices/system/cpu/cpu10/topology/core_siblings:0000 /sys/devices/system/cpu/cpu10/topology/core_siblings_list: /sys/devices/system/cpu/cpu10/topology/physical_package_id:0 /sys/devices/system/cpu/cpu10/topology/thread_siblings:0000 /sys/devices/system/cpu/cpu10/topology/thread_siblings_list: I did some testing with and without BOOTPARAM_HOTPLUG_CPU0 enabled, and up'd and down'd threads in sequence, randomly, by thread group, by socket group and didn't see any issues. core_siblings and thread_siblings are "numa siblings that are online" and "thread siblings that are online" and are used as such within the kernel. They must be zero'd out when the thread is offline. CONFIG_PERMANENT_CPU_TOPOLOGY=y changes the lifetime of the topology directory from existing when a thread is online to when a thread is created and destroyed. Signed-off-by: Prarit Bhargava <pra...@redhat.com> Cc: Thomas Gleixner <t...@linutronix.de> Cc: Ingo Molnar <mi...@redhat.com> Cc: "H. Peter Anvin" <h...@zytor.com> Cc: x...@kernel.org Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org> Cc: Peter Zijlstra <pet...@infradead.org> Cc: Len Brown <len.br...@intel.com> Cc: Borislav Petkov <b...@suse.de> Cc: Andi Kleen <a...@linux.intel.com> Cc: Jiri Olsa <jo...@redhat.com> Cc: Juergen Gross <jgr...@suse.com> --- arch/x86/kernel/smpboot.c | 3 --- drivers/base/Kconfig | 12 ++++++++++++ drivers/base/cpu.c | 8 ++++++++ 3 files changed, 20 insertions(+), 3 deletions(-) diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c index 4296beb8fdd3..ae82f8f45b61 100644 --- a/arch/x86/kernel/smpboot.c +++ b/arch/x86/kernel/smpboot.c @@ -1472,7 +1472,6 @@ static void recompute_smt_state(void) static void remove_siblinginfo(int cpu) { int sibling; - struct cpuinfo_x86 *c = &cpu_data(cpu); for_each_cpu(sibling, topology_core_cpumask(cpu)) { cpumask_clear_cpu(cpu, topology_core_cpumask(sibling)); @@ -1490,8 +1489,6 @@ static void remove_siblinginfo(int cpu) cpumask_clear(cpu_llc_shared_mask(cpu)); cpumask_clear(topology_sibling_cpumask(cpu)); cpumask_clear(topology_core_cpumask(cpu)); - c->phys_proc_id = 0; - c->cpu_core_id = 0; cpumask_clear_cpu(cpu, cpu_sibling_setup_mask); recompute_smt_state(); } diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig index 98504ec99c7d..b3935a272c3c 100644 --- a/drivers/base/Kconfig +++ b/drivers/base/Kconfig @@ -324,4 +324,16 @@ config CMA_ALIGNMENT endif +config PERMANENT_CPU_TOPOLOGY + bool "Permanent CPU Topology" + depends on HOTPLUG_CPU + def_bool y if X86_64 + help + This option configures CPU topology to be permanent for the lifetime + of the CPU (until it is physically removed). Selecting Y here + results in the kernel reporting the physical location for offlined + CPUs. + + If unsure, leave the default value as is. + endmenu diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c index c59cecbb651a..e4e5d0cfe363 100644 --- a/drivers/base/cpu.c +++ b/drivers/base/cpu.c @@ -262,6 +262,7 @@ static struct attribute_group topology_attr_group = { .name = "topology" }; +#ifndef CONFIG_PERMANENT_CPU_TOPOLOGY /* Add/Remove cpu_topology interface for CPU device */ static int topology_add_dev(unsigned int cpu) { @@ -318,11 +319,15 @@ out: } device_initcall(topology_sysfs_init); +#endif static const struct attribute_group *common_cpu_attr_groups[] = { #ifdef CONFIG_KEXEC &crash_note_cpu_attr_group, #endif +#ifdef CONFIG_PERMANENT_CPU_TOPOLOGY + &topology_attr_group, +#endif NULL }; @@ -330,6 +335,9 @@ static const struct attribute_group *hotplugable_cpu_attr_groups[] = { #ifdef CONFIG_KEXEC &crash_note_cpu_attr_group, #endif +#ifdef CONFIG_PERMANENT_CPU_TOPOLOGY + &topology_attr_group, +#endif NULL }; -- 1.7.9.3