counter — SMP-friendly kernel counter implementation

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

Debian                                            March 6, 2020                                       COUNTER(9)

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 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.

       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");

       The counter facility first appeared in FreeBSD 10.0.

       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_PCPUuma(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.

       counter — SMP-friendly kernel counter implementation

atomic(9), critical(9), locking(9), malloc(9), ratecheck(9), sysctl(9), SYSINIT(9), uma(9)

#include<sys/types.h>#include<sys/systm.h>#include<sys/counter.h>counter_u64_tcounter_u64_alloc(intwait);

       voidcounter_u64_free(counter_u64_tc);

       voidcounter_u64_add(counter_u64_tc, int64_tv);

       voidcounter_enter();

       voidcounter_exit();

       voidcounter_u64_add_protected(counter_u64_tc, int64_tv);

       uint64_tcounter_u64_fetch(counter_u64_tc);

       voidcounter_u64_zero(counter_u64_tc);

       int64_tcounter_ratecheck(structcounter_rate*cr, int64_tlimit);

       COUNTER_U64_SYSINIT(counter_u64_tc);

       COUNTER_U64_DEFINE_EARLY(counter_u64_tc);

       #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);