Ubuntu Manpage: counter — SMP-friendly kernel counter implementation

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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);

       #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.

counter_u64_free(c)
Free the previously allocated counter c.

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 to safely update 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.

SYSCTL_COUNTER_U64(parent, nbr, name, access, ptr, descr)
Declare a static sysctl 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 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 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 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");

HISTORY

       The counter facility first appeared in FreeBSD 10.0.

AUTHORS

       The counter facility was written by Gleb Smirnoff and Konstantin Belousov.

Debian                                           March 14, 2016                                       COUNTER(9)