The kernel traces various actions by writing records to an internal buffer, which can later be retrieved and printed.
The kernel trace format is described in the ktrace.h and ktrace-def.h files under system/ulib/zircon-internal/include/lib/zircon-internal.
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
system/ulib/zircon-internal/include/lib/zircon-internal/ktrace.h. The default is 0xfff which traces everything.
What to trace can also be controlled by the
ktrace command-line utility, described below.
The size of the trace buffer is fixed at boot time and is controlled by the
ktrace.bufsize kernel command-line parameter. Its value is the buffer size in megabytes. The default is 32MB.
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.
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.
Fuchsia‘s tracing system supports collecting kernel trace records through the
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.