Surprisingly there is no straight way to get number of processors. Sysfs attributes /sys/devices/system/cpu/online, "present" and "possible" shows cpus bitmap as comma-separated list of numbers and ranges. This format is human-freindly but it's hard to parse and unusable for scripts.
This patchs adds new attributes into /sys/devices/system/cpu/ nr_online, nr_present, nr_possible, last_online, last_present, last_possible. nr_* shows count of cpus as plain decimal number. last_* shows highest index, it might not equal to NR-1 if bitmap is sparse. This might be useful for preallocating arrays for ranges of cpu indexes. Signed-off-by: Konstantin Khlebnikov <koc...@gmail.com> --- Currently GNU C Library does weird things in the implementation of get_nprocs() and get_nprocs_conf() (sysconf(_SC_NPROCESSORS_ONLN / _SC_NPROCESSORS_CONF)). get_nprocs() parses "/sys/devices/system/cpu/online", counts lines started with "cpu" in "/proc/stat" and even parses "/proc/cpuinfo". And because all this is expensive it caches result for one second. get_nprocs_conf() counts subdirs in /sys/devices/system/cpu named "cpu*" It also parses "/proc/cpuinfo" on alpha and sparc. This function returns count of present CPUs, probably it should return number of possible CPUs otherwise userspace isn't able to handle cpu-hotplug. This should be common situation for virtual-machines like XEN or KVM. If nothing works well both functions return 1. They cannot report error. For example hotspot JVM uses sysconf(_SC_NPROCESSORS_CONF) to detect UP machine where it can drop 'lock' prefix in atomic operations. If sysfs and proc aren't available it crashes or hangs inside GC =) [ http://hg.openjdk.java.net/jdk8/jdk8/hotspot/file/87ee5ee27509/src/os/linux/vm/os_linux.cpp#l291 ] Other user which I've found is userspace-RCU. It uses per-cpu arrays indexed by getcpu() and uses sysconf(_SC_NPROCESSORS_CONF) - 1 as last possible cpu. So this doesn't work for sparse bitmap, also I'm not sure about cpu-hotplug. --- Documentation/ABI/testing/sysfs-devices-system-cpu | 17 +++++++++ drivers/base/cpu.c | 37 +++++++++++++++++++- 2 files changed, 53 insertions(+), 1 deletion(-) diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu index d5a0d33..e05edb7 100644 --- a/Documentation/ABI/testing/sysfs-devices-system-cpu +++ b/Documentation/ABI/testing/sysfs-devices-system-cpu @@ -37,6 +37,23 @@ Description: CPU topology files that describe kernel limits related to See Documentation/cputopology.txt for more information. +What: /sys/devices/system/cpu/nr_online + /sys/devices/system/cpu/nr_present + /sys/devices/system/cpu/nr_possible + /sys/devices/system/cpu/last_online + /sys/devices/system/cpu/last_present + /sys/devices/system/cpu/last_possible +Date: May 2014 +Contact: Linux kernel mailing list <linux-kernel@vger.kernel.org> +Description: Number and last index of processors in the system + + nr_{online,possible,present}: count of cpus + + last_{online,possible,present}: highest cpu index + + See also /sys/devices/system/cpu/{online,possible,present} above + + What: /sys/devices/system/cpu/probe /sys/devices/system/cpu/release Date: November 2009 diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c index 006b1bc..1ca73c7 100644 --- a/drivers/base/cpu.c +++ b/drivers/base/cpu.c @@ -214,14 +214,43 @@ static ssize_t show_cpus_attr(struct device *dev, return n; } +static ssize_t show_nr_cpus_attr(struct device *dev, + struct device_attribute *attr, char *buf) +{ + struct cpu_attr *ca = container_of(attr, struct cpu_attr, attr); + + return snprintf(buf, PAGE_SIZE, "%u\n", cpumask_weight(*ca->map)); +} + +static ssize_t show_last_cpus_attr(struct device *dev, + struct device_attribute *attr, char *buf) +{ + struct cpu_attr *ca = container_of(attr, struct cpu_attr, attr); + unsigned last_cpu = find_last_bit(cpumask_bits(*ca->map), NR_CPUS); + + return snprintf(buf, PAGE_SIZE, "%u\n", last_cpu); +} + #define _CPU_ATTR(name, map) \ { __ATTR(name, 0444, show_cpus_attr, NULL), map } -/* Keep in sync with cpu_subsys_attrs */ +#define _NR_CPU_ATTR(name, map) \ + { __ATTR(name, 0444, show_nr_cpus_attr, NULL), map } + +#define _LAST_CPU_ATTR(name, map) \ + { __ATTR(name, 0444, show_last_cpus_attr, NULL), map } + +/* Keep in sync with cpu_root_attrs */ static struct cpu_attr cpu_attrs[] = { _CPU_ATTR(online, &cpu_online_mask), _CPU_ATTR(possible, &cpu_possible_mask), _CPU_ATTR(present, &cpu_present_mask), + _NR_CPU_ATTR(nr_online, &cpu_online_mask), + _NR_CPU_ATTR(nr_possible, &cpu_possible_mask), + _NR_CPU_ATTR(nr_present, &cpu_present_mask), + _LAST_CPU_ATTR(last_online, &cpu_online_mask), + _LAST_CPU_ATTR(last_possible, &cpu_possible_mask), + _LAST_CPU_ATTR(last_present, &cpu_present_mask), }; /* @@ -378,6 +407,12 @@ static struct attribute *cpu_root_attrs[] = { &cpu_attrs[0].attr.attr, &cpu_attrs[1].attr.attr, &cpu_attrs[2].attr.attr, + &cpu_attrs[3].attr.attr, + &cpu_attrs[4].attr.attr, + &cpu_attrs[5].attr.attr, + &cpu_attrs[6].attr.attr, + &cpu_attrs[7].attr.attr, + &cpu_attrs[8].attr.attr, &dev_attr_kernel_max.attr, &dev_attr_offline.attr, #ifdef CONFIG_GENERIC_CPU_AUTOPROBE -- 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/
