NAME
kernhist
—
basic low-level kernel history tracing
mechanism
SYNOPSIS
options KERNHIST
#include <sys/kernhist.h>
Below are the functions and macros provided by kernhist.h:
KERNHIST_DECL
(name);
KERNHIST_DEFINE
(name);
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_FUNC
(fname);
KERNHIST_DUMP
(struct
kern_history name);
void
kernhist_dump
(struct
kern_history *history);
void
kernhist_dumpmask
(u_int32_t
bitmask);
void
kernhist_print
(void
(*pr)(const char *, ...));
DESCRIPTION
Thekernhist
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 integers. 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 corresponding 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:
KERNHIST_DECL
(name)- Declare an extern struct kern_history name.
KERNHIST_DEFINE
(name)- Define a struct kern_history name.
KERNHIST_INIT
(name, num_entries)- Dynamically initialise a kernel history called name with num_entries entries.
KERNHIST_INITIALIZER
(name, buffer)- Initialise a statically defined kernel history called name using buffer as a static allocation used for the buffer.
KERNHIST_INIT_STATIC
(name, buffer)- Initialise a statically declared kernel history name, using the statically allocated buffer for history entries.
KERNHIST_FUNC
(fname)- 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 arguments in the history as a unique event.
KERNHIST_CALLED
(name)- 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 called. KERNHIST_CALLARGS
(name, fmt, arg0, arg1, arg2, arg3)- A combination of
KERNHIST_CALLED
() andKERNHIST_LOG
() that avoids having a “called!” log message in addition to a message containing normal arguments with a format string. KERNHIST_DUMP
(name)- Call
kernhist_dump
() on the named kernel history. kernhist_dump
(history)- Dump the entire contents of the specified kernel history.
kernhist_dumpmask
(bitmask)- Used to dump a well known list of kernel histories. The following
histories and their respective value (as seen in
kernhist.h) are available:
- 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”.
kernhist_print
(pr)- Print all the kernel histories to the kernel message buffer. The
pr
() argument is currently ignored.
CODE REFERENCES
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
sysctl(9) mechanism.
SEE ALSO
HISTORY
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.
AUTHORS
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.
BUGS
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 release.
In addition to a statically-defined set of kernel histories, it would be possible to allow modular code to register and unregister their own histories dynamically, when a module is loaded or unloaded.
The kernhist_print
() function currently
ignores its pr argument.