linux-kernel - Re: [PATCH 0/3] [RFC] init, tracing: Add initcall trace events

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Message-ID: <CAEi0qNnm0uor5T7KWVmA9sUg21FJkvP0HWzCPcRz5QzO4M5Fjw@mail.gmail.com>
Date:   Mon, 26 Mar 2018 18:50:32 -0700
From:   "Joel Fernandes (Google)" <joel.opensrc@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Abderrahmane Benbachir <abderrahmane.benbachir@...ymtl.ca>,
        AmÃ©rico Wang <xiyou.wangcong@...il.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 0/3] [RFC] init, tracing: Add initcall trace events

Hi Steve,

On Fri, Mar 23, 2018 at 8:02 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> A while ago we had a boot tracer. But it was eventually removed:
>  commit 30dbb20e68e6f ("tracing: Remove boot tracer").
>
> The rational 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.
>
> 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     [000] ...1     0.000000: initcall_level: level=console
> <idle>-0     [000] ...1     0.000000: initcall_start: func=con_init+0x0/0x224
> <idle>-0     [000] ...1     0.000000: initcall_finish: func=con_init+0x0/0x224 ret=0
> <idle>-0     [000] ...1     0.000000: initcall_start: func=hvc_console_init+0x0/0x19
> <idle>-0     [000] ...1     0.000000: initcall_finish: func=hvc_console_init+0x0/0x19 ret=0
> <idle>-0     [000] ...1     0.000000: initcall_start: func=xen_cons_init+0x0/0x60
> <idle>-0     [000] ...1     0.000000: initcall_finish: func=xen_cons_init+0x0/0x60 ret=0
> <idle>-0     [000] ...1     0.000000: initcall_start: func=univ8250_console_init+0x0/0x2d
> <idle>-0     [000] ...1     0.000000: initcall_finish: func=univ8250_console_init+0x0/0x2d ret=0

Will this make initcall_debug not work if CONFIG_TRACEPOINTS is turned
off? Although it builds but I think this initcall_debug feature will
fail, maybe CONFIG_TRACEPOINTS should be selected somewhere?

I recently ran into some issues like this for my preemptirq
tracepoints patch (which I will post again soon :D) where lockdep
needed the tracepoints and I had to select it.

thanks,

- Joel