The kernel traces various actions by writing records to an internal buffer, which can later be retrieved and printed.
Kernel trace format
The kernel trace format is described in the ktrace.h and ktrace-def.h files under system/ulib/zircon-internal/include/lib/zircon-internal.
Controlling what to trace
Control of what to trace is provided by a kernel command-line parameter
ktrace.grpmask. The value is specified as 0xNNN and is a bitmask
of tracing groups to enable. See the *KTRACE_GRP_* values in
The default is 0xfff, which traces everything.
What to trace can also be controlled by the
ktrace command-line utility,
Trace buffer size
The size of the trace buffer is fixed at boot time and is controlled by
ktrace.bufsize kernel command-line parameter. Its value is the
buffer size in megabytes. The default is 32MB.
ktrace command-line utility
Kernel tracing may be controlled with the
ktrace command-line utility.
$ ktrace --help Usage: ktrace [options] <control> Where <control> is one of: start <group_mask> - start tracing stop - stop tracing rewind - rewind trace buffer written - print bytes written to trace buffer Note: This value doesn't reset on "rewind". Instead, the rewind takes effect on the next "start". save <path> - save contents of trace buffer to <path> Options: --help - Duh.
Pretty-printing a kernel trace
The host tool
ktrace-dump can be used to pretty-print a kernel trace.
First collect the trace on the target:
$ ktrace start 0xfff ... do something ... $ ktrace stop $ ktrace save /tmp/save.ktrace
Then copy the file to the development host, and dump it:
host$ out/default/host-tools/netcp :/tmp/save.ktrace save.ktrace host$ out/default/host-tools/ktrace-dump save.ktrace > save.dump
The pretty-printed output can be quite voluminous, thus it's recommended to send it to a file and then view it in your editor or whatever.
Use with Fuchsia Tracing
Fuchsia's tracing system supports collecting kernel trace records through
ktrace_provider trace provider.
For documentation of Fuchsia's tracing system see the documentation in
Fuchsia tracing system.
More information on
ktrace can be found in the
full list of kernel command line parameters.