Here is a neat ftrace recipe for tracing execution while the Linux kernel is inside a particular function. This helps when a kernel function or its children are failing but you don't know where or why.
ftrace will trigger on particular functions if you give it set_graph_function values. That way you only see traces from the functions you are interested in. This eliminates the noise you get when tracing all function entries/returns without a filter.
Let's trace virtio_dev_probe() and all its children:
echo virtio_dev_probe >/sys/kernel/debug/tracing/set_graph_function echo function_graph >/sys/kernel/debug/tracing/current_tracer echo 1 >/sys/kernel/debug/tracing/tracing_on modprobe transport_virtio echo 0 >/sys/kernel/debug/tracing/tracing_on echo >/sys/kernel/debug/tracing/current_tracer echo >/sys/kernel/debug/tracing/set_graph_function cat /sys/kernel/debug/tracing/trace
Here is some example output:
...
0) | virtqueue_kick [virtio_ring]() {
0) + 30.207 us | virtqueue_kick_prepare [virtio_ring]();
0) + 13.342 us | vp_notify [virtio_pci]();
0) + 90.315 us | }
0) # 61946.45 us | }
0) 1.046 us | mutex_unlock();
0) # 102833.9 us | }
0) 2.411 us | vp_get_status [virtio_pci]();
0) 0.826 us | vp_get_status [virtio_pci]();
0) ! 130.773 us | vp_set_status [virtio_pci]();
0) | virtio_config_enable [virtio]() {
0) 0.689 us | _raw_spin_lock_irq();
0) + 33.796 us | }
0) # 105349.9 us | }
I haven't figured out whether set_graph_function can be used on functions whose kernel module has not been loaded yet. I think the answer is no, but please let me know in the comments if there is a way to do it.
