| |
| 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_event_open() |
| system call: |
| |
| int sys_perf_event_open(struct perf_event_attr *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_event_attr' is: |
| |
| struct perf_event_attr { |
| /* |
| * The MSB of the config word signifies if the rest contains cpu |
| * specific (raw) counter configuration data, if unset, the next |
| * 7 bits are an event type and the rest of the bits are the event |
| * identifier. |
| */ |
| __u64 config; |
| |
| __u64 irq_period; |
| __u32 record_type; |
| __u32 read_format; |
| |
| __u64 disabled : 1, /* off by default */ |
| 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 */ |
| mmap : 1, /* include mmap data */ |
| munmap : 1, /* include munmap data */ |
| comm : 1, /* include comm data */ |
| |
| __reserved_1 : 52; |
| |
| __u32 extra_config_len; |
| __u32 wakeup_events; /* wakeup every n events */ |
| |
| __u64 __reserved_2; |
| __u64 __reserved_3; |
| }; |
| |
| The '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_ffff_ffff_ffff |
| |
| 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_event_open() syscall: |
| */ |
| enum hw_event_ids { |
| /* |
| * Common hardware events, generalized by the kernel: |
| */ |
| PERF_COUNT_HW_CPU_CYCLES = 0, |
| PERF_COUNT_HW_INSTRUCTIONS = 1, |
| PERF_COUNT_HW_CACHE_REFERENCES = 2, |
| PERF_COUNT_HW_CACHE_MISSES = 3, |
| PERF_COUNT_HW_BRANCH_INSTRUCTIONS = 4, |
| PERF_COUNT_HW_BRANCH_MISSES = 5, |
| PERF_COUNT_HW_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_SW_CPU_CLOCK = 0, |
| PERF_COUNT_SW_TASK_CLOCK = 1, |
| PERF_COUNT_SW_PAGE_FAULTS = 2, |
| PERF_COUNT_SW_CONTEXT_SWITCHES = 3, |
| PERF_COUNT_SW_CPU_MIGRATIONS = 4, |
| PERF_COUNT_SW_PAGE_FAULTS_MIN = 5, |
| PERF_COUNT_SW_PAGE_FAULTS_MAJ = 6, |
| PERF_COUNT_SW_ALIGNMENT_FAULTS = 7, |
| PERF_COUNT_SW_EMULATION_FAULTS = 8, |
| }; |
| |
| Counters of the type PERF_TYPE_TRACEPOINT are available when the ftrace event |
| tracer is available, and event_id values can be obtained from |
| /debug/tracing/events/*/*/id |
| |
| |
| 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. |
| |
| |
| A read() on a counter returns the current value of the counter and possible |
| additional values as specified by 'read_format', each value is a u64 (8 bytes) |
| in size. |
| |
| /* |
| * Bits that can be set in hw_event.read_format to request that |
| * reads on the counter should return the indicated quantities, |
| * in increasing order of bit value, after the counter value. |
| */ |
| enum perf_event_read_format { |
| PERF_FORMAT_TOTAL_TIME_ENABLED = 1, |
| PERF_FORMAT_TOTAL_TIME_RUNNING = 2, |
| }; |
| |
| Using these additional values one can establish the overcommit ratio for a |
| particular counter allowing one to take the round-robin scheduling effect |
| into account. |
| |
| |
| 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. The record_type controls what data is recorded on each |
| interrupt: |
| |
| /* |
| * Bits that can be set in hw_event.record_type to request information |
| * in the overflow packets. |
| */ |
| enum perf_event_record_format { |
| PERF_RECORD_IP = 1U << 0, |
| PERF_RECORD_TID = 1U << 1, |
| PERF_RECORD_TIME = 1U << 2, |
| PERF_RECORD_ADDR = 1U << 3, |
| PERF_RECORD_GROUP = 1U << 4, |
| PERF_RECORD_CALLCHAIN = 1U << 5, |
| }; |
| |
| Such (and other) events will be recorded in a ring-buffer, which is |
| available to user-space using mmap() (see below). |
| |
| 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 '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 'mmap' and 'munmap' bits allow recording of PROT_EXEC mmap/munmap |
| operations, these can be used to relate userspace IP addresses to actual |
| code, even after the mapping (or even the whole process) is gone, |
| these events are recorded in the ring-buffer (see below). |
| |
| The 'comm' bit allows tracking of process comm data on process creation. |
| This too is recorded in the ring-buffer (see below). |
| |
| The 'pid' parameter to the perf_event_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_event_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. |
| |
| |
| Like stated, asynchronous events, like counter overflow or PROT_EXEC mmap |
| tracking are logged into a ring-buffer. This ring-buffer is created and |
| accessed through mmap(). |
| |
| The mmap size should be 1+2^n pages, where the first page is a meta-data page |
| (struct perf_event_mmap_page) that contains various bits of information such |
| as where the ring-buffer head is. |
| |
| /* |
| * Structure of the page that can be mapped via mmap |
| */ |
| struct perf_event_mmap_page { |
| __u32 version; /* version number of this structure */ |
| __u32 compat_version; /* lowest version this is compat with */ |
| |
| /* |
| * Bits needed to read the hw counters in user-space. |
| * |
| * u32 seq; |
| * s64 count; |
| * |
| * do { |
| * seq = pc->lock; |
| * |
| * barrier() |
| * if (pc->index) { |
| * count = pmc_read(pc->index - 1); |
| * count += pc->offset; |
| * } else |
| * goto regular_read; |
| * |
| * barrier(); |
| * } while (pc->lock != seq); |
| * |
| * NOTE: for obvious reason this only works on self-monitoring |
| * processes. |
| */ |
| __u32 lock; /* seqlock for synchronization */ |
| __u32 index; /* hardware counter identifier */ |
| __s64 offset; /* add to hardware counter value */ |
| |
| /* |
| * Control data for the mmap() data buffer. |
| * |
| * User-space reading this value should issue an rmb(), on SMP capable |
| * platforms, after reading this value -- see perf_event_wakeup(). |
| */ |
| __u32 data_head; /* head in the data section */ |
| }; |
| |
| NOTE: the hw-counter userspace bits are arch specific and are currently only |
| implemented on powerpc. |
| |
| The following 2^n pages are the ring-buffer which contains events of the form: |
| |
| #define PERF_RECORD_MISC_KERNEL (1 << 0) |
| #define PERF_RECORD_MISC_USER (1 << 1) |
| #define PERF_RECORD_MISC_OVERFLOW (1 << 2) |
| |
| struct perf_event_header { |
| __u32 type; |
| __u16 misc; |
| __u16 size; |
| }; |
| |
| enum perf_event_type { |
| |
| /* |
| * The MMAP events record the PROT_EXEC mappings so that we can |
| * correlate userspace IPs to code. They have the following structure: |
| * |
| * struct { |
| * struct perf_event_header header; |
| * |
| * u32 pid, tid; |
| * u64 addr; |
| * u64 len; |
| * u64 pgoff; |
| * char filename[]; |
| * }; |
| */ |
| PERF_RECORD_MMAP = 1, |
| PERF_RECORD_MUNMAP = 2, |
| |
| /* |
| * struct { |
| * struct perf_event_header header; |
| * |
| * u32 pid, tid; |
| * char comm[]; |
| * }; |
| */ |
| PERF_RECORD_COMM = 3, |
| |
| /* |
| * When header.misc & PERF_RECORD_MISC_OVERFLOW the event_type field |
| * will be PERF_RECORD_* |
| * |
| * struct { |
| * struct perf_event_header header; |
| * |
| * { u64 ip; } && PERF_RECORD_IP |
| * { u32 pid, tid; } && PERF_RECORD_TID |
| * { u64 time; } && PERF_RECORD_TIME |
| * { u64 addr; } && PERF_RECORD_ADDR |
| * |
| * { u64 nr; |
| * { u64 event, val; } cnt[nr]; } && PERF_RECORD_GROUP |
| * |
| * { u16 nr, |
| * hv, |
| * kernel, |
| * user; |
| * u64 ips[nr]; } && PERF_RECORD_CALLCHAIN |
| * }; |
| */ |
| }; |
| |
| NOTE: PERF_RECORD_CALLCHAIN is arch specific and currently only implemented |
| on x86. |
| |
| Notification of new events is possible through poll()/select()/epoll() and |
| fcntl() managing signals. |
| |
| Normally a notification is generated for every page filled, however one can |
| additionally set perf_event_attr.wakeup_events to generate one every |
| so many counter overflow events. |
| |
| Future work will include a splice() interface to the ring-buffer. |
| |
| |
| 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_EVENT_IOC_ENABLE); |
| |
| or disabled with |
| |
| ioctl(fd, PERF_EVENT_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. |
| |
| Additionally, non-inherited overflow counters can use |
| |
| ioctl(fd, PERF_EVENT_IOC_REFRESH, nr); |
| |
| to enable a counter for 'nr' events, after which it gets disabled again. |
| |
| A process can enable or disable all the counter groups that are |
| attached to it, using prctl: |
| |
| prctl(PR_TASK_PERF_EVENTS_ENABLE); |
| |
| prctl(PR_TASK_PERF_EVENTS_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. |
| |
| |
| Arch requirements |
| ----------------- |
| |
| If your architecture does not have hardware performance metrics, you can |
| still use the generic software counters based on hrtimers for sampling. |
| |
| So to start with, in order to add HAVE_PERF_EVENTS to your Kconfig, you |
| will need at least this: |
| - asm/perf_event.h - a basic stub will suffice at first |
| - support for atomic64 types (and associated helper functions) |
| - set_perf_event_pending() implemented |
| |
| If your architecture does have hardware capabilities, you can override the |
| weak stub hw_perf_event_init() to register hardware counters. |
| |
| Architectures that have d-cache aliassing issues, such as Sparc and ARM, |
| should select PERF_USE_VMALLOC in order to avoid these for perf mmap(). |