A while ago we had a boot tracer. But it was eventually removed: commit 30dbb20e68e6f ("tracing: Remove boot tracer").
The rationale was because there is already a initcall_debug boot option that causes printk()s of all the initcall functions. The problem with the initcall_debug option is that printk() is awfully slow, and makes it difficult to see the real impact of initcalls. Mainly because a single printk() is usually slower than most initcall functions. Yes the timings are done within the printks, but it slows down the boot up tremendously. Tracing can do the same without needing to slow down boot up. Instead of bringing back the boot tracer, adding trace events around the initcall functions, and even one to denote which level the initcall functions are being called from, adds the necessary information to analyze the initcalls without the high overhead of printk()s, that can substantially slow down the boot process. Another positive, is that the console initcall functions themselves can also be traced. The timestamps are not operational at that time but you can see which consoles are being registered. I saw this on one of my test boxes: <idle>-0  ...1 0.000000: initcall_level: level=console <idle>-0  ...1 0.000000: initcall_start: func=con_init+0x0/0x224 <idle>-0  ...1 0.000000: initcall_finish: func=con_init+0x0/0x224 ret=0 <idle>-0  ...1 0.000000: initcall_start: func=hvc_console_init+0x0/0x19 <idle>-0  ...1 0.000000: initcall_finish: func=hvc_console_init+0x0/0x19 ret=0 <idle>-0  ...1 0.000000: initcall_start: func=xen_cons_init+0x0/0x60 <idle>-0  ...1 0.000000: initcall_finish: func=xen_cons_init+0x0/0x60 ret=0 <idle>-0  ...1 0.000000: initcall_start: func=univ8250_console_init+0x0/0x2d <idle>-0  ...1 0.000000: initcall_finish: func=univ8250_console_init+0x0/0x2d ret=0 I didn't even realize that I had some of those consoles configured. Anyone have any issues with me adding this? git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git ftrace/core Head SHA1: 7af49b07cb525d36a916b841dc9329c187789e1f Abderrahmane Benbachir (1): init, tracing: instrument security and console initcall trace events Steven Rostedt (VMware) (3): init, tracing: Add initcall trace events init, tracing: Have printk come through the trace events for initcall_debug init: Have initcall_debug still work without CONFIG_TRACEPOINTS ---- Changes since v1: Added the last patch that makes initcall_debug work even when TRACEPOINTS is not configured. It just goes back to the old method (with if statements, instead of hooking into trace_events). include/trace/events/initcall.h | 66 ++++++++++++++++++++++++++++++++++ init/main.c | 78 ++++++++++++++++++++++++++++++++--------- kernel/printk/printk.c | 7 +++- security/security.c | 8 ++++- 4 files changed, 141 insertions(+), 18 deletions(-) create mode 100644 include/trace/events/initcall.h