COUNTERS_ALLOC(9) FreeBSD 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_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 <
[email protected]>.
FreeBSD 14.1-RELEASE-p8 September 16, 2023 FreeBSD 14.1-RELEASE-p8