The Zircon kernel's internal tracing system can be active on boot. This means that one can trace at least the kernel side of booting by setting a boot option. Then, once booting has finished, the data is already there, one just needs to collect it.
Enable the Kernel Tracing Boot Parameter
The size of the kernel's trace buffer can be changed at boot time
ktrace.bufsize=N command line option, where
N is the size
of the buffer in megabytes.
The choice of data to collect is controlled with the `ktrace.grpmask=0xNNN' command line option. The 0xNNN value is a bit mask of KTRACE_GRP_* values from //zircon/kernel/lib/boot-options/include/lib/boot-options/options.inc. The default is 0x000, which disables all trace categories (or groups in ktrace parlance).
Set the boot option as a GN variable via your
fx set command:
fx set ... --args='dev_kernel_cmdline=["ktrace.grpmask=0xFFF"]'
You'll then need to rebuild and redeploy.
Including kernel boot trace data in trace results
Once you enable the kernel tracing boot parameter, as long as the kernel's internal trace buffer is
not rewound, after boot, the data is available to be included in the trace. This is achieved by
kernel:retain to the
ffx trace or
trace program. Note that the moment a trace
is made without passing
kernel:retain then the ktrace buffer is rewound and the data is lost.
ffx trace start --categories "kernel,kernel:retain" --buffer-size 32 --duration 1
There are a few important things to note here.
The first thing to note is the categories passed:
kernel category tells the kernel to trace everything.
In this example the kernel has already been tracing everything: that is
the default on boot. It is specified here as a simple way to
ktrace_provider, which is the interface between the Fuchsia tracing
system and the kernel, that kernel data is being collected.
kernel:retain category tells
ktrace_provider not to rewind the
kernel trace buffer at the start of tracing.
The second is the buffer size. The kernel's default trace buffer size is 32MB whereas the Fuchsia trace default buffer size is 4MB. Using a larger Fuchsia trace buffer size means there is enough space to hold the contents of the kernel's trace buffer.
The third important thing to note is that in this example we just want to grab the current contents of the trace buffer, and aren't interested in tracing anything more. That is why a duration of one second is used.