man/man9/counter.9

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.Dd March 11, 2021
.Dt COUNTER 9
.Os
.Sh NAME
.Nm counter
.Nd "SMP-friendly kernel counter implementation"
.Sh SYNOPSIS
.In sys/types.h
.In sys/systm.h
.In sys/counter.h
.Ft counter_u64_t
.Fn counter_u64_alloc "int wait"
.Ft void
.Fn counter_u64_free "counter_u64_t c"
.Ft void
.Fn counter_u64_add "counter_u64_t c" "int64_t v"
.Ft void
.Fn counter_enter
.Ft void
.Fn counter_exit
.Ft void
.Fn counter_u64_add_protected "counter_u64_t c" "int64_t v"
.Ft uint64_t
.Fn counter_u64_fetch "counter_u64_t c"
.Ft void
.Fn counter_u64_zero "counter_u64_t c"
.Ft int64_t
.Fn counter_ratecheck "struct counter_rate *cr" "int64_t limit"
.Fn COUNTER_U64_SYSINIT "counter_u64_t c"
.Fn COUNTER_U64_DEFINE_EARLY "counter_u64_t c"
.In sys/sysctl.h
.Fn SYSCTL_COUNTER_U64 parent nbr name access ptr descr
.Fn SYSCTL_ADD_COUNTER_U64 ctx parent nbr name access ptr descr
.Fn SYSCTL_COUNTER_U64_ARRAY parent nbr name access ptr len descr
.Fn SYSCTL_ADD_COUNTER_U64_ARRAY ctx parent nbr name access ptr len descr
.Sh DESCRIPTION
.Nm
is a generic facility to create counters
that can be utilized for any purpose (such as collecting statistical
data).
A
.Nm
is guaranteed to be lossless when several kernel threads do simultaneous
updates.
However,
.Nm
does not block the calling thread,
also no
.Xr atomic 9
operations are used for the update, therefore the counters
can be used in any non-interrupt context.
Moreover,
.Nm
has special optimisations for SMP environments, making
.Nm
update faster than simple arithmetic on the global variable.
Thus
.Nm
is considered suitable for accounting in the performance-critical
code paths.
.Bl -tag -width indent
.It Fn counter_u64_alloc wait
Allocate a new 64-bit unsigned counter.
The
.Fa wait
argument is the
.Xr malloc 9
wait flag, should be either
.Va M_NOWAIT
or
.Va M_WAITOK .
If
.Va M_NOWAIT
is specified the operation may fail and return
.Dv NULL .
.It Fn counter_u64_free c
Free the previously allocated counter
.Fa c .
It is safe to pass
.Dv NULL .
.It Fn counter_u64_add c v
Add
.Fa v
to
.Fa c .
The KPI does not guarantee any protection from wraparound.
.It Fn counter_enter
Enter mode that would allow the safe update of several counters via
.Fn counter_u64_add_protected .
On some machines this expands to
.Xr critical 9
section, while on other is a nop.
See
.Sx IMPLEMENTATION DETAILS .
.It Fn counter_exit
Exit mode for updating several counters.
.It Fn counter_u64_add_protected c v
Same as
.Fn counter_u64_add ,
but should be preceded by
.Fn counter_enter .
.It Fn counter_u64_fetch c
Take a snapshot of counter
.Fa c .
The data obtained is not guaranteed to reflect the real cumulative
value for any moment.
.It Fn counter_u64_zero c
Clear the counter
.Fa c
and set it to zero.
.It Fn counter_ratecheck cr limit
The function is a multiprocessor-friendly version of
.Fn ppsratecheck
which uses
.Nm
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
.Fn counter_ratecheck
returns number of events since previous reset.
.It Fn COUNTER_U64_SYSINIT c
Define a
.Xr SYSINIT 9
initializer for the global counter
.Fa c .
.It Fn COUNTER_U64_DEFINE_EARLY c
Define and initialize a global counter
.Fa c .
It is always safe to increment
.Fa c ,
though updates prior to the
.Dv SI_SUB_COUNTER
.Xr SYSINIT 9
event are lost.
.It Fn SYSCTL_COUNTER_U64 parent nbr name access ptr descr
Declare a static
.Xr sysctl 9
oid that would represent a
.Nm .
The
.Fa ptr
argument should be a pointer to allocated
.Vt counter_u64_t .
A read of the oid returns value obtained through
.Fn counter_u64_fetch .
Any write to the oid zeroes it.
.It Fn SYSCTL_ADD_COUNTER_U64 ctx parent nbr name access ptr descr
Create a
.Xr sysctl 9
oid that would represent a
.Nm .
The
.Fa ptr
argument should be a pointer to allocated
.Vt counter_u64_t .
A read of the oid returns value obtained through
.Fn counter_u64_fetch .
Any write to the oid zeroes it.
.It Fn SYSCTL_COUNTER_U64_ARRAY parent nbr name access ptr len descr
Declare a static
.Xr sysctl 9
oid that would represent an array of
.Nm .
The
.Fa ptr
argument should be a pointer to allocated array of
.Vt counter_u64_t's .
The
.Fa len
argument should specify number of elements in the array.
A read of the oid returns len-sized array of
.Vt uint64_t
values  obtained through
.Fn counter_u64_fetch .
Any write to the oid zeroes all array elements.
.It Fn SYSCTL_ADD_COUNTER_U64_ARRAY ctx parent nbr name access ptr len descr
Create a
.Xr sysctl 9
oid that would represent an array of
.Nm .
The
.Fa ptr
argument should be a pointer to allocated array of
.Vt counter_u64_t's .
The
.Fa len
argument should specify number of elements in the array.
A read of the oid returns len-sized array of
.Vt uint64_t
values obtained through
.Fn counter_u64_fetch .
Any write to the oid zeroes all array elements.
.El
.Sh IMPLEMENTATION DETAILS
On all architectures
.Nm
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
.Va UMA_ZONE_PCPU
.Xr 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.
.Pp
On amd64 a
.Nm 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.
.Pp
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.
.Pp
On some architectures updating a counter require a
.Xr critical 9
section.
.Sh EXAMPLES
The following example creates a static counter array exported to
userspace through a sysctl:
.Bd -literal -offset indent
#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");
.Ed
.Sh SEE ALSO
.Xr atomic 9 ,
.Xr critical 9 ,
.Xr locking 9 ,
.Xr malloc 9 ,
.Xr ratecheck 9 ,
.Xr sysctl 9 ,
.Xr SYSINIT 9 ,
.Xr uma 9
.Sh HISTORY
The
.Nm
facility first appeared in
.Fx 10.0 .
.Sh AUTHORS
.An -nosplit
The
.Nm
facility was written by
.An Gleb Smirnoff
and
.An Konstantin Belousov .