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.Dd November 30, 2008
.Dt KTR 9
.Os
.Sh NAME
.Nm CTR0 , CTR1 , CTR2 , CTR3 , CTR4 , CTR5
.Nd kernel tracing facility
.Sh SYNOPSIS
.In sys/param.h
.In sys/ktr.h
.Vt "extern int ktr_cpumask" ;
.Vt "extern int ktr_entries" ;
.Vt "extern int ktr_extend" ;
.Vt "extern int ktr_mask" ;
.Vt "extern int ktr_verbose" ;
.Vt "extern struct ktr_entry ktr_buf[]" ;
.Ft void
.Fn CTR0 "u_int mask" "char *format"
.Ft void
.Fn CTR1 "u_int mask" "char *format" "arg1"
.Ft void
.Fn CTR2 "u_int mask" "char *format" "arg1" "arg2"
.Ft void
.Fn CTR3 "u_int mask" "char *format" "arg1" "arg2" "arg3"
.Ft void
.Fn CTR4 "u_int mask" "char *format" "arg1" "arg2" "arg3" "arg4"
.Ft void
.Fn CTR5 "u_int mask" "char *format" "arg1" "arg2" "arg3" "arg4" "arg5"
.Ft void
.Fn CTR6 "u_int mask" "char *format" "arg1" "arg2" "arg3" "arg4" "arg5" "arg6"
.Sh DESCRIPTION
KTR provides a circular buffer of events that can be logged in a
.Xr printf 9
style
fashion.
These events can then be dumped with
.Xr ddb 4 ,
.Xr gdb 1
or
.Xr ktrdump 8 .
.Pp
Events are created and logged in the kernel via the
.Dv CTR Ns Ar x
macros.
The first parameter is a mask of event types
.Pq Dv KTR_*
defined in
.In sys/ktr.h .
The event will be logged only if any of the event types specified in
.Fa mask
are enabled in the global event mask stored in
.Va ktr_mask .
The
.Fa format
argument is a
.Xr printf 9
style format string used to build the text of the event log message.
Following the
.Fa format
string are zero to five arguments referenced by
.Fa format .
Each event is logged with a file name and source line number of the
originating CTR call, and a timestamp in addition to the log message.
.Pp
The event is stored in the circular buffer with supplied arguments as is,
and formatting is done at the dump time.
Do not use pointers to the objects with limited lifetime, for instance,
strings, because the pointer may become invalid when buffer is printed.
.Pp
Note that the different macros differ only in the number of arguments each
one takes, as indicated by its name.
.Pp
The
.Va ktr_entries
variable contains the number of entries in the
.Va ktr_buf
array.
These variables are mostly useful for post-mortem crash dump tools to locate
the base of the circular trace buffer and its length.
.Pp
The
.Va ktr_mask
variable contains the run time mask of events to log.
.Pp
The CPU event mask is stored in the
.Va ktr_cpumask
variable.
.Pp
The
.Va ktr_verbose
variable stores the verbose flag that controls whether events are logged to
the console in addition to the event buffer.
.Sh EXAMPLES
This example demonstrates the use of tracepoints at the
.Dv KTR_PROC
logging level.
.Bd -literal
void
mi_switch()
{
	...
	/*
	 * Pick a new current process and record its start time.
	 */
	...
	CTR3(KTR_PROC, "mi_switch: old proc %p (pid %d)", p, p->p_pid);
	...
	cpu_switch();
	...
	CTR3(KTR_PROC, "mi_switch: new proc %p (pid %d)", p, p->p_pid);
	...
}
.Ed
.Sh SEE ALSO
.Xr ktr 4 ,
.Xr ktrdump 8
.Sh HISTORY
The KTR kernel tracing facility first appeared in
.Bsx 3.0
and was imported into
.Fx 5.0 .
.Sh BUGS
Currently there is one global buffer shared among all CPUs.
It might be profitable at some point in time to use per-CPU buffers instead
so that if one CPU halts or starts spinning, then the log messages it
emitted just prior to halting or spinning will not be drowned out by events
from the other CPUs.
.Pp
The arguments given in
.Fn CTRx
macros are stored as
.Vt u_long ,
so do not pass arguments larger than size of an
.Vt u_long
type.
For example passing 64bit arguments on 32bit architectures will give incorrect
results.