Performance Counters for Linux ------------------------------ Performance counters are special hardware registers available on most modern CPUs. These registers count the number of certain types of hw events: such as instructions executed, cachemisses suffered, or branches mis-predicted - without slowing down the kernel or applications. These registers can also trigger interrupts when a threshold number of events have passed - and can thus be used to profile the code that runs on that CPU. The Linux Performance Counter subsystem provides an abstraction of these hardware capabilities. It provides per task and per CPU counters, counter groups, and it provides event capabilities on top of those. It provides "virtual" 64-bit counters, regardless of the width of the underlying hardware counters. Performance counters are accessed via special file descriptors. There's one file descriptor per virtual counter used. The special file descriptor is opened via the perf_counter_open() system call: int sys_perf_counter_open(struct perf_counter_hw_event *hw_event_uptr, pid_t pid, int cpu, int group_fd, unsigned long flags); The syscall returns the new fd. The fd can be used via the normal VFS system calls: read() can be used to read the counter, fcntl() can be used to set the blocking mode, etc. Multiple counters can be kept open at a time, and the counters can be poll()ed. When creating a new counter fd, 'perf_counter_hw_event' is: /* * Event to monitor via a performance monitoring counter: */ struct perf_counter_hw_event { __u64 event_config; __u64 irq_period; __u64 record_type; __u64 read_format; __u64 disabled : 1, /* off by default */ nmi : 1, /* NMI sampling */ inherit : 1, /* children inherit it */ pinned : 1, /* must always be on PMU */ exclusive : 1, /* only group on PMU */ exclude_user : 1, /* don't count user */ exclude_kernel : 1, /* ditto kernel */ exclude_hv : 1, /* ditto hypervisor */ exclude_idle : 1, /* don't count when idle */ __reserved_1 : 55; __u32 extra_config_len; __u32 __reserved_4; __u64 __reserved_2; __u64 __reserved_3; }; The 'event_config' field specifies what the counter should count. It is divided into 3 bit-fields: raw_type: 1 bit (most significant bit) 0x8000_0000_0000_0000 type: 7 bits (next most significant) 0x7f00_0000_0000_0000 event_id: 56 bits (least significant) 0x00ff_0000_0000_0000 If 'raw_type' is 1, then the counter will count a hardware event specified by the remaining 63 bits of event_config. The encoding is machine-specific. If 'raw_type' is 0, then the 'type' field says what kind of counter this is, with the following encoding: enum perf_event_types { PERF_TYPE_HARDWARE = 0, PERF_TYPE_SOFTWARE = 1, PERF_TYPE_TRACEPOINT = 2, }; A counter of PERF_TYPE_HARDWARE will count the hardware event specified by 'event_id': /* * Generalized performance counter event types, used by the hw_event.event_id * parameter of the sys_perf_counter_open() syscall: */ enum hw_event_ids { /* * Common hardware events, generalized by the kernel: */ PERF_COUNT_CPU_CYCLES = 0, PERF_COUNT_INSTRUCTIONS = 1, PERF_COUNT_CACHE_REFERENCES = 2, PERF_COUNT_CACHE_MISSES = 3, PERF_COUNT_BRANCH_INSTRUCTIONS = 4, PERF_COUNT_BRANCH_MISSES = 5, PERF_COUNT_BUS_CYCLES = 6, }; These are standardized types of events that work relatively uniformly on all CPUs that implement Performance Counters support under Linux, although there may be variations (e.g., different CPUs might count cache references and misses at different levels of the cache hierarchy). If a CPU is not able to count the selected event, then the system call will return -EINVAL. More hw_event_types are supported as well, but they are CPU-specific and accessed as raw events. For example, to count "External bus cycles while bus lock signal asserted" events on Intel Core CPUs, pass in a 0x4064 event_id value and set hw_event.raw_type to 1. A counter of type PERF_TYPE_SOFTWARE will count one of the available software events, selected by 'event_id': /* * Special "software" counters provided by the kernel, even if the hardware * does not support performance counters. These counters measure various * physical and sw events of the kernel (and allow the profiling of them as * well): */ enum sw_event_ids { PERF_COUNT_CPU_CLOCK = 0, PERF_COUNT_TASK_CLOCK = 1, PERF_COUNT_PAGE_FAULTS = 2, PERF_COUNT_CONTEXT_SWITCHES = 3, PERF_COUNT_CPU_MIGRATIONS = 4, PERF_COUNT_PAGE_FAULTS_MIN = 5, PERF_COUNT_PAGE_FAULTS_MAJ = 6, }; Counters come in two flavours: counting counters and sampling counters. A "counting" counter is one that is used for counting the number of events that occur, and is characterised by having irq_period = 0 and record_type = PERF_RECORD_SIMPLE. A read() on a counting counter simply returns the current value of the counter as an 8-byte number. A "sampling" counter is one that is set up to generate an interrupt every N events, where N is given by 'irq_period'. A sampling counter has irq_period > 0 and record_type != PERF_RECORD_SIMPLE. The record_type controls what data is recorded on each interrupt, and the available values are currently: /* * IRQ-notification data record type: */ enum perf_counter_record_type { PERF_RECORD_SIMPLE = 0, PERF_RECORD_IRQ = 1, PERF_RECORD_GROUP = 2, }; A record_type value of PERF_RECORD_IRQ will record the instruction pointer (IP) at which the interrupt occurred. A record_type value of PERF_RECORD_GROUP will record the event_config and counter value of all of the other counters in the group, and should only be used on a group leader (see below). Currently these two values are mutually exclusive, but record_type will become a bit-mask in future and support other values. A sampling counter has an event queue, into which an event is placed on each interrupt. A read() on a sampling counter will read the next event from the event queue. If the queue is empty, the read() will either block or return an EAGAIN error, depending on whether the fd has been set to non-blocking mode or not. The 'disabled' bit specifies whether the counter starts out disabled or enabled. If it is initially disabled, it can be enabled by ioctl or prctl (see below). The 'nmi' bit specifies, for hardware events, whether the counter should be set up to request non-maskable interrupts (NMIs) or normal interrupts. This bit is ignored if the user doesn't have CAP_SYS_ADMIN privilege (i.e. is not root) or if the CPU doesn't generate NMIs from hardware counters. The 'inherit' bit, if set, specifies that this counter should count events on descendant tasks as well as the task specified. This only applies to new descendents, not to any existing descendents at the time the counter is created (nor to any new descendents of existing descendents). The 'pinned' bit, if set, specifies that the counter should always be on the CPU if at all possible. It only applies to hardware counters and only to group leaders. If a pinned counter cannot be put onto the CPU (e.g. because there are not enough hardware counters or because of a conflict with some other event), then the counter goes into an 'error' state, where reads return end-of-file (i.e. read() returns 0) until the counter is subsequently enabled or disabled. The 'exclusive' bit, if set, specifies that when this counter's group is on the CPU, it should be the only group using the CPU's counters. In future, this will allow sophisticated monitoring programs to supply extra configuration information via 'extra_config_len' to exploit advanced features of the CPU's Performance Monitor Unit (PMU) that are not otherwise accessible and that might disrupt other hardware counters. The 'exclude_user', 'exclude_kernel' and 'exclude_hv' bits provide a way to request that counting of events be restricted to times when the CPU is in user, kernel and/or hypervisor mode. The 'pid' parameter to the perf_counter_open() system call allows the counter to be specific to a task: pid == 0: if the pid parameter is zero, the counter is attached to the current task. pid > 0: the counter is attached to a specific task (if the current task has sufficient privilege to do so) pid < 0: all tasks are counted (per cpu counters) The 'cpu' parameter allows a counter to be made specific to a CPU: cpu >= 0: the counter is restricted to a specific CPU cpu == -1: the counter counts on all CPUs (Note: the combination of 'pid == -1' and 'cpu == -1' is not valid.) A 'pid > 0' and 'cpu == -1' counter is a per task counter that counts events of that task and 'follows' that task to whatever CPU the task gets schedule to. Per task counters can be created by any user, for their own tasks. A 'pid == -1' and 'cpu == x' counter is a per CPU counter that counts all events on CPU-x. Per CPU counters need CAP_SYS_ADMIN privilege. The 'flags' parameter is currently unused and must be zero. The 'group_fd' parameter allows counter "groups" to be set up. A counter group has one counter which is the group "leader". The leader is created first, with group_fd = -1 in the perf_counter_open call that creates it. The rest of the group members are created subsequently, with group_fd giving the fd of the group leader. (A single counter on its own is created with group_fd = -1 and is considered to be a group with only 1 member.) A counter group is scheduled onto the CPU as a unit, that is, it will only be put onto the CPU if all of the counters in the group can be put onto the CPU. This means that the values of the member counters can be meaningfully compared, added, divided (to get ratios), etc., with each other, since they have counted events for the same set of executed instructions. Counters can be enabled and disabled in two ways: via ioctl and via prctl. When a counter is disabled, it doesn't count or generate events but does continue to exist and maintain its count value. An individual counter or counter group can be enabled with ioctl(fd, PERF_COUNTER_IOC_ENABLE); or disabled with ioctl(fd, PERF_COUNTER_IOC_DISABLE); Enabling or disabling the leader of a group enables or disables the whole group; that is, while the group leader is disabled, none of the counters in the group will count. Enabling or disabling a member of a group other than the leader only affects that counter - disabling an non-leader stops that counter from counting but doesn't affect any other counter. A process can enable or disable all the counter groups that are attached to it, using prctl: prctl(PR_TASK_PERF_COUNTERS_ENABLE); prctl(PR_TASK_PERF_COUNTERS_DISABLE); This applies to all counters on the current process, whether created by this process or by another, and doesn't affect any counters that this process has created on other processes. It only enables or disables the group leaders, not any other members in the groups.