COUNTER(9) FreeBSD Kernel Developer's Manual COUNTER(9)
NAME
counter - SMP-friendly kernel counter implementation
SYNOPSIS
#include <sys/types.h>
#include <sys/systm.h>
#include <sys/counter.h>
counter_u64_t
counter_u64_alloc(int wait);
void
counter_u64_free(counter_u64_t c);
void
counter_u64_add(counter_u64_t c, int64_t v);
void
counter_enter();
void
counter_exit();
void
counter_u64_add_protected(counter_u64_t c, int64_t v);
uint64_t
counter_u64_fetch(counter_u64_t c);
void
counter_u64_zero(counter_u64_t c);
int64_t
counter_ratecheck(struct counter_rate *cr, int64_t limit);
COUNTER_U64_SYSINIT(counter_u64_t c);
COUNTER_U64_DEFINE_EARLY(counter_u64_t c);
#include <sys/sysctl.h>
SYSCTL_COUNTER_U64(parent, nbr, name, access, ptr, descr);
SYSCTL_ADD_COUNTER_U64(ctx, parent, nbr, name, access, ptr, descr);
SYSCTL_COUNTER_U64_ARRAY(parent, nbr, name, access, ptr, len, descr);
SYSCTL_ADD_COUNTER_U64_ARRAY(ctx, parent, nbr, name, access, ptr, len,
descr);
DESCRIPTION
counter is a generic facility to create counters that can be utilized for
any purpose (such as collecting statistical data). A counter is
guaranteed to be lossless when several kernel threads do simultaneous
updates. However, counter does not block the calling thread, also no
atomic(9) operations are used for the update, therefore the counters can
be used in any non-interrupt context. Moreover, counter has special
optimisations for SMP environments, making counter update faster than
simple arithmetic on the global variable. Thus counter is considered
suitable for accounting in the performance-critical code paths.
counter_u64_alloc(wait)
Allocate a new 64-bit unsigned counter. The wait argument is the
malloc(9) wait flag, should be either M_NOWAIT or M_WAITOK. If
M_NOWAIT is specified the operation may fail and return NULL.
counter_u64_free(c)
Free the previously allocated counter c. It is safe to pass
NULL.
counter_u64_add(c, v)
Add v to c. The KPI does not guarantee any protection from
wraparound.
counter_enter()
Enter mode that would allow the safe update of several counters
via counter_u64_add_protected(). On some machines this expands
to critical(9) section, while on other is a nop. See
IMPLEMENTATION DETAILS.
counter_exit()
Exit mode for updating several counters.
counter_u64_add_protected(c, v)
Same as counter_u64_add(), but should be preceded by
counter_enter().
counter_u64_fetch(c)
Take a snapshot of counter c. The data obtained is not
guaranteed to reflect the real cumulative value for any moment.
counter_u64_zero(c)
Clear the counter c and set it to zero.
counter_ratecheck(cr, limit)
The function is a multiprocessor-friendly version of
ppsratecheck() which uses counter internally. Returns non-
negative value if the rate is not yet reached during the current
second, and a negative value otherwise. If the limit was reached
on previous second, but was just reset back to zero, then
counter_ratecheck() returns number of events since previous
reset.
COUNTER_U64_SYSINIT(c)
Define a SYSINIT(9) initializer for the global counter c.
COUNTER_U64_DEFINE_EARLY(c)
Define and initialize a global counter c. It is always safe to
increment c, though updates prior to the SI_SUB_COUNTER
SYSINIT(9) event are lost.
SYSCTL_COUNTER_U64(parent, nbr, name, access, ptr, descr)
Declare a static sysctl(9) oid that would represent a counter.
The ptr argument should be a pointer to allocated counter_u64_t.
A read of the oid returns value obtained through
counter_u64_fetch(). Any write to the oid zeroes it.
SYSCTL_ADD_COUNTER_U64(ctx, parent, nbr, name, access, ptr, descr)
Create a sysctl(9) oid that would represent a counter. The ptr
argument should be a pointer to allocated counter_u64_t. A read
of the oid returns value obtained through counter_u64_fetch().
Any write to the oid zeroes it.
SYSCTL_COUNTER_U64_ARRAY(parent, nbr, name, access, ptr, len, descr)
Declare a static sysctl(9) oid that would represent an array of
counter. The ptr argument should be a pointer to allocated array
of counter_u64_t's. The len argument should specify number of
elements in the array. A read of the oid returns len-sized array
of uint64_t values obtained through counter_u64_fetch(). Any
write to the oid zeroes all array elements.
SYSCTL_ADD_COUNTER_U64_ARRAY(ctx, parent, nbr, name, access, ptr, len,
descr)
Create a sysctl(9) oid that would represent an array of counter.
The ptr argument should be a pointer to allocated array of
counter_u64_t's. The len argument should specify number of
elements in the array. A read of the oid returns len-sized array
of uint64_t values obtained through counter_u64_fetch(). Any
write to the oid zeroes all array elements.
IMPLEMENTATION DETAILS
On all architectures counter is implemented using per-CPU data fields
that are specially aligned in memory, to avoid inter-CPU bus traffic due
to shared use of the variables between CPUs. These are allocated using
UMA_ZONE_PCPU uma(9) zone. The update operation only touches the field
that is private to current CPU. Fetch operation loops through all per-
CPU fields and obtains a snapshot sum of all fields.
On amd64 a counter update is implemented as a single instruction without
lock semantics, operating on the private data for the current CPU, which
is safe against preemption and interrupts.
On i386 architecture, when machine supports the cmpxchg8 instruction,
this instruction is used. The multi-instruction sequence provides the
same guarantees as the amd64 single-instruction implementation.
On some architectures updating a counter require a critical(9) section.
EXAMPLES
The following example creates a static counter array exported to
userspace through a sysctl:
#define MY_SIZE 8
static counter_u64_t array[MY_SIZE];
SYSCTL_COUNTER_U64_ARRAY(_debug, OID_AUTO, counter_array, CTLFLAG_RW,
&array[0], MY_SIZE, "Test counter array");
SEE ALSO
atomic(9), critical(9), locking(9), malloc(9), ratecheck(9), sysctl(9),
SYSINIT(9), uma(9)
HISTORY
The counter facility first appeared in FreeBSD 10.0.
AUTHORS
The counter facility was written by Gleb Smirnoff and Konstantin
Belousov.
FreeBSD 13.1-RELEASE-p6 March 11, 2021 FreeBSD 13.1-RELEASE-p6
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