COUNTERS_ALLOC(9) | Kernel Developer's Manual | COUNTERS_ALLOC(9)

# NAME

`counters_alloc`, `counters_free`, `COUNTERS_BOOT_MEMORY`, `COUNTERS_BOOT_INITIALIZER`, `counters_alloc_ncpus`, `counters_enter`, `counters_leave`, `counters_inc`, `counters_add`, `counters_pkt`, `counters_read`, `counters_zero` — per CPU counters

# SYNOPSIS

`#include <sys/percpu.h>`

struct cpumem *
`counters_alloc`(unsigned int ncounters);

void
`counters_free`(struct cpumem *cm, unsigned int ncounters);

`COUNTERS_BOOT_MEMORY`(NAME, unsigned int ncounters);

`COUNTERS_BOOT_INITIALIZER`(NAME);

struct cpumemt *
`counters_alloc_ncpus`(struct cpumem *cm, unsigned int ncounters);

uint64_t *
`counters_enter`(struct counters_ref *ref, struct cpumem *cm);

void
`counters_leave`(struct counters_ref *ref, struct cpumem *cm);

void
`counters_inc`(struct cpumem *cm, unsigned int counter);

void
`counters_add`(struct cpumem *cm, unsigned int counter, uint64_t v);

void
`counters_pkt`(struct cpumem *cm, unsigned int pcounter, unsigned int bcounter, uint64_t bytes);

void
`counters_read`(struct cpumem *cm, uint64_t *counters, unsigned int ncounters, uint64_t *scratch);

void
`counters_zero`(struct cpumem *cm, unsigned int ncounters);

# DESCRIPTION

The per CPU counter API builds on the per CPU memory API and provides access to a series of uint64_t counter values on each CPU.

The API provides versioning of counter updates without using interlocked CPU instructions so they can all be read in a consistent state. Updates to counters should be limited to addition or subtraction of uint64_t values.

An alternate implementation of the API is provided on uni-processor systems (i.e. when the kernel is not built with `MULTIPROCESSOR` defined) that provides no overhead compared to direct access to a data structure. This allows the API to be used without affecting the performance of uni-processor systems.

`counters_alloc`() allocates memory for a series of uint64_t values on each CPU. ncounters specifies the number of counters to be allocated. The counters will be zeroed on allocation.

`counters_free`() deallocates each CPU's counters. The same ncounters argument originally provided to `counters_alloc`() must be passed to `counters_free`().

`counters_alloc`() may only be used after all the CPUs in the system have been attached. If a set of CPU counters needs to be available during early boot, a cpumem pointer and counters for the boot CPU may be statically allocated.

`COUNTERS_BOOT_MEMORY`() statically allocates a set of counter for use on the boot CPU before the other CPUs in the system have been attached. The allocation is identified by NAME and provides memory for the number of counters specified by ncounters. The counters will be initialised to zero.

`COUNTERS_BOOT_INITIALIZER`() is used to initialise a cpumem pointer with the memory that was previously allocated using `COUNTERS_BOOT_MEMORY`() and identified by NAME.

`counters_alloc_ncpus`() allocates additional sets of counters for the CPUs that were attached during boot. The cpumem structure cm must have been initialised with `COUNTERS_BOOT_INITIALIZER`(). The same number of counters originally passed to COUNTERS_BOOT_MEMORY must be specified by ncounters. The counters on the boot CPU will be preserved, while the counters for the additional CPUs will be zeroed on allocation.

Counters that have been allocated with `COUNTERS_BOOT_MEMORY`() and `counters_alloc_ncpus`() cannot be deallocated with counters_free.

`counters_enter`() provides access to the current CPU's set of counters referenced by cm. The caller's reference to the counters is held by ref.

`counters_leave`() indicates the end of access to the current CPU's set of counters referenced by cm. The reference held by ref is released by this call.

`counters_inc`() increments the counter at the index specified by counter in the array of counters referenced by cm.

`counters_add`() adds the value of v to the counter at the index specified by counter in the array of counters referenced by cm.

`counters_pkt`() increments the value at the index specified by pcounter and adds the value of bytes to the counter at the index specified by bcounter in the array of counters referenced by cm.

`counters_read`() iterates over each CPU's set of counters referenced by cm, takes a consistent snapshot of the counters, and then sums them together. The sum of the counters is written to the counters array. The number of counters is specified with ncounters. scratch may point to a buffer used to temporarily hold counters. If `NULL`, one will be dynamically allocated and freed.

`counters_zero`() iterates over each CPU's set of counters referenced by cm and zeroes them. The number of counters is specified with ncounters. `counters_zero`() itself does not prevent concurrent updates of the counters; it is up to the caller to serialise this call with other actions.

# CONTEXT

`counters_alloc`(), `counters_free`(), `counters_alloc_ncpus`(), and `counters_read`() may be called during autoconf, or from process context.

`counters_enter`(), `counters_leave`(), `counters_inc`(), `counters_add`(), `counters_pkt`(), and `counters_zero`() may be called during autoconf, from process context, or from interrupt context. The per CPU counters API does not provide any locking or serialisation of access to each CPU's set of counters beyond isolating each CPU's update. It is up to the caller to provide appropriate locking or serialisation around calls to these functions to prevent concurrent access to the relevant data structures.

# RETURN VALUES

`counters_alloc`() and `counters_alloc_ncpus`() will return an opaque cpumem pointer that references each CPU's set of counters.

`counters_enter`() returns a reference to the current CPU's set of counters.

# EXAMPLES

The following is an example of providing per CPU counters at boot time based on the mbuf(9) statistics code in sys/kern/uipc_mbuf.c.

    /* mbuf stats */
    COUNTERS_BOOT_MEMORY(mbstat_boot, MBSTAT_COUNT);
    struct cpumem *mbstat = COUNTERS_BOOT_INITIALIZER(mbstat_boot);

    /*
     * this function is called from init_main.c after devices
     * (including additional CPUs) have been attached
     */
    void
    mbcpuinit()
    {
    	mbstat = counters_alloc_ncpus(mbstat, MBSTAT_COUNT);
    }

    struct mbuf *
    m_get(int nowait, int type)
    {
    	...

            struct counters_ref cr;
            uint64_t *counters;
            int s;

    	...

            s = splnet();
            counters = counters_enter(&cr, mbstat);
            counters[type]++;
            counters_leave(&cr, mbstat);
            splx(s);

    	...
    }

    struct mbuf *
    m_free(struct mbuf *m)
    {
    	...

            struct counters_ref cr;
            uint64_t *counters;
            int s;

    	...

            s = splnet();
            counters = counters_enter(&cr, mbstat);
            counters[m->m_type]--;
            counters_leave(&cr, mbstat);
            splx(s);

    	...
    }

# SEE ALSO

cpumem_get(9), malloc(9)

# HISTORY

The per CPU counter API first appeared in OpenBSD 6.1.

# AUTHORS

The per CPU counter API was written by David Gwynne <dlg@openbsd.org>.

September 16, 2023 | Debian