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);
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 implemention of the API is provided on uni-processor (i.e. when the kernel is not built with MULTIPROCESSOR defined) systems that provides no overhead compared to direct access to a data structure. This allows the API to be used without affecting the performance 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. Any attempt to do so will lead to undefined behaviour.

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.

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>.