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KERNHIST(9) NetBSD Kernel Developer's Manual KERNHIST(9)
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kernhist -- basic low-level kernel history tracing mechanism
Below are the functions and macros provided by kernhist.h:
KERNHIST_INIT(name, unsigned num_entries);
KERNHIST_INITIALIZER(name, void *buffer);
KERNHIST_INIT_STATIC(struct kern_history name, void *buffer);
KERNHIST_LOG(struct kern_history name, const char *fmt, u_long arg0,
u_long arg1, u_long arg2, u_long arg3);
KERNHIST_CALLARGS(struct kern_history name, const char *fmt, u_long arg0,
u_long arg1, u_long arg2, u_long arg3);
KERNHIST_CALLED(struct kern_history name);
KERNHIST_DUMP(struct kern_history name);
kernhist_dump(struct kern_history *history);
kernhist_print(void (*pr)(const char *, ...));
The kernhist facility provides a very low-level tracing facility that can
be called extremely early in the kernel initialisation. It provides a
simple restricted printf(3) format syntax with a maximum of 4 arguments,
each of type uintmax_t.
options KERNHIST must be present in the kernel configuration to enable
these functions and macros.
A kernel history is a fixed-size buffer, either statically or dynamically
allocated, that is written and read on a circular basis. Each entry
includes the time the entry was made, the CPU from which the entry was
recorded, the printf(3) like format string and length, the function name
and length, the unique call count for this function, and the 4 arguments.
The history event data can be viewed using the -U and -u histname options
to vmstat(1), or by using the show kernhist command in ddb(4). User-
written programs can retrieve history data from the kernel using the
sysctl(9) variable kern.hist.histname.
The format string must be a literal string that can be referenced later
as it is not stored with the event (only a pointer to the format string
is stored). It should only contain conversion specifiers suitable for
uintmax_t sized values, such as ``%jx'', ``%ju'', and ``%jo'', and
address (pointer) arguments should be cast to uintptr_t to avoid compiler
errors on architectures where pointers are smaller than uintmax_t inte-
gers. Conversion specifiers without a length modifier, and specifiers
with length modifiers other than j, should not be used.
Conversion specifiers that require additional dereferences of their cor-
responding arguments, such as ``%s'', will not work in vmstat(1), but
will work when called from ddb(4).
These macros provide access to most kernel history functionality:
Declare an extern struct kern_history name.
Define a struct kern_history name.
Dynamically initialise a kernel history called name with
Initialise a statically defined kernel history called name using
buffer as a static allocation used for the buffer.
Initialise a statically declared kernel history name, using the
statically allocated buffer for history entries.
Declare necessary variables for kernhist to be used this function.
Callable only once per function.
KERNHIST_LOG(name, fmt, arg0, arg1, arg2, arg3)
For the given kernel history name, log the format string and argu-
ments in the history as a unique event.
Declare a function as being called. Either this or
KERNHIST_CALLARGS() must be used once, near the function entry
point, to maintain the number of times the function has been
KERNHIST_CALLARGS(name, fmt, arg0, arg1, arg2, arg3)
A combination of KERNHIST_CALLED() and KERNHIST_LOG() that avoids
having a ``called!'' log message in addition to a message contain-
ing normal arguments with a format string.
Call kernhist_dump() on the named kernel history.
Dump the entire contents of the specified kernel history.
Used to dump a well known list of kernel histories. The following
histories and their respective value (as seen in kernhist.h) are
KERNHIST_UVMMAPHIST Include events from ``maphist''.
KERNHIST_UVMPDHIST Include events from ``pdhist''.
KERNHIST_UVMUBCHIST Include events from ``ubchist''.
KERNHIST_UVMLOANHIST Include events from ``loanhist''.
KERNHIST_USBHIST Include events from ``usbhist''.
KERNHIST_SCDEBUGHIST Include events from ``scdebughist''.
KERNHIST_BIOHIST Include events from ``biohist''.
Print all the kernel histories to the kernel message buffer. The
pr() argument is currently ignored.
The kernhist functionality is implemented within the files
sys/sys/kernhist.h and sys/kern/kern_history.c. The former file contains
the definitions of data structures used to export the data via the
vmstat(1), usbdi(9), uvm(9)
A uvm-specific version of the kernhist facility first appeared in
NetBSD 1.4. The generalized version of kernhist appeared in NetBSD 6.0.
The sysctl(9) interface to kernhist was introduced in NetBSD 8.0.
kernhist was originally written by Charles D. Cranor as part of the
uvm(9) framework, under the name UVMHIST. Matthew R. Green generalized
it into its current form to be available to non uvm(9) frameworks. Paul
Goyette <pgoyette@NetBSD.org> provided the sysctl(9) interface.
The restriction against using ``%s'' printf(3) specifier in format
strings could be reduced to literal strings (such as the table of system
call names) if vmstat(1) was extended to convert ``%s'' strings into user
addresses after copying the strings out.
KERNHIST_FUNC() could be converted to use __func__ always, as all the
callers already do.
The kernhist_dumpmask() list of masks could be properly published and
made available, and as such this function may be removed in a future
In addition to a statically-defined set of kernel histories, it would be
possible to allow modular code to register and unregister their own his-
tories dynamically, when a module is loaded or unloaded.
The kernhist_print() function currently ignores its pr argument.
NetBSD 9.1 October 25, 2017 NetBSD 9.1