linux-kernel - Re: x86_64 Compiler Output Kernel Bloat v4.4

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Message-ID: <CAO6TR8Xe46XcSN+iBLYGTk+xW9DJK0HarBCPLND8C56u1T7c6A@mail.gmail.com>
Date:	Tue, 19 Jan 2016 08:32:26 -0700
From:	Jeff Merkey <linux.mdb@...il.com>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...nel.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>, Jiri Olsa <jolsa@...nel.org>
Subject: Re: x86_64 Compiler Output Kernel Bloat v4.4

On 1/19/16, Jiri Olsa <jolsa@...hat.com> wrote:
> On Mon, Jan 18, 2016 at 02:45:14PM -0700, Jeff Merkey wrote:
>
> SNIP
>
>> > 0xffffffffa073e019 E8C2579FE0      call
>> > touch_softlockup_watchdog_sync
>> > 0xffffffffa073e01e E87D2B9BE0      call   clocksource_touch_watchdog
>> > 0xffffffffa073e023 E8F8099AE0      call   rcu_cpu_stall_reset
>> > 0xffffffffa073e028 E893579FE0      call   touch_nmi_watchdog
>> > 0xffffffffa073e02d 5D              pop    rbp
>> > 0xffffffffa073e02e 6690            data16
>> > 0xffffffffa073e030 C3              ret
>> > 0xffffffffa073e031 6666666666662E0F1F840000000000  nop    cs:WORD PTR
>> > [rax+rax]=0x0000
>> > mdb|mdb:
>> > 0xffffffffa073e040 0F1F440000      nop    DWORD PTR [rax+rax]=0x0  <<
>> > 0xffffffffa073e045 55              push   rbp
>> > 0xffffffffa073e046 4889E5          mov    rbp,rsp
>> > 0xffffffffa073e049 4157            push   r15
>> > 0xffffffffa073e04b 4156            push   r14
>> > 0xffffffffa073e04d 4989FE          mov    r14,rdi
>> > 0xffffffffa073e050 4155            push   r13
>> > (0)> g
>> >
>> > Jeff
>> >
>>
>> I think xor eax,eax is a lot shorter and fewer bytes.
>
> that 5 byte nop gets replaced by call instruction once the
> function trace is enabled
>
> explained in CONFIG_FUNCTION_TRACER option doc:
>
>
> config FUNCTION_TRACER
>         bool "Kernel Function Tracer"
>         depends on HAVE_FUNCTION_TRACER
>         select KALLSYMS
>         select GENERIC_TRACER
>         select CONTEXT_SWITCH_TRACER
>         help
>           Enable the kernel to trace every kernel function. This is done
>           by using a compiler feature to insert a small, 5-byte
> No-Operation
>           instruction at the beginning of every kernel function, which NOP
>           sequence is then dynamically patched into a tracer call when
>           tracing is enabled by the administrator. If it's runtime disabled
>           (the bootup default), then the overhead of the instructions is
> very
>           small and not measurable even in micro-benchmarks.
>
> jirka
>

Takes up a lot of space since linux has so monay small functions.
Someone sent me the ftrace stuff and I looked over it.  Thanks.

Jeff