| # Recording a boot trace |
| |
| 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 |
| with the `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: |
| |
| ```posix-terminal |
| fx set ... --args='dev_kernel_cmdline=["ktrace.grpmask=0xFFF"]' |
| ``` |
| |
| You'll then need to rebuild and redeploy. |
| |
| For more information on Zircon command line options see: |
| - [kernel_cmdline](/docs/reference/kernel/kernel_cmdline.md) |
| - [kernel_build](/docs/development/kernel/build.md) |
| |
| ## 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 |
| passing category `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. |
| |
| Example: |
| |
| ```posix-terminal |
| 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` and `kernel:retain`. |
| The `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 |
| tell `ktrace_provider`, which is the interface between the Fuchsia tracing |
| system and the kernel, that kernel data is being collected. |
| The `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. |