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Message-ID: <CAO6TR8XV8XdN1+eJJYMTXBpQNbeXt_kWiFQ5OjJutn_EH9CRhA@mail.gmail.com> Date: Mon, 18 Jan 2016 14:34:00 -0700 From: Jeff Merkey <linux.mdb@...il.com> To: LKML <linux-kernel@...r.kernel.org> Cc: 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 1/18/16, Jeff Merkey <linux.mdb@...il.com> wrote: > Hi, > > I noticed that in the assembler output for the x86_64 builds almost > every single function originating from C code has a nop instruction > that prefaces the function call. I guess the concern with this is > the wasted space issue as each one of these placeholders takes up a > bunch of bytes at the head of each function. Is there a reason this > assembler header is there in the first place to anyones knowledge? > Since every single function just about is prefaced by this inert 5 > byte instruction it adds up to quite a bit of bloat in the size of the > linux executable. > > 0xffffffffa073e010 0F1F440000 nop DWORD PTR [rax+rax]=0x0 > > The intel assembler format shows the bytes that comprise each > instruction. The GDB format does not. Both are provided. > > 0xffffffffa073e050 4155 push r13 > (0)> id mdb_watchdogs > mdb|mdb_watchdogs: > 0xffffffffa073e010 mdb_watchdogs: nopl 0x0(%rax,%rax,1)) << > 0xffffffffa073e015 mdb_watchdogs+0x5: push %rbp > 0xffffffffa073e016 mdb_watchdogs+0x6: mov %rsp,%rbp > 0xffffffffa073e019 mdb_watchdogs+0x9: callq 0xffffffff811337e0 > touch_softlockup_watchdog_sync > 0xffffffffa073e01e mdb_watchdogs+0xe: callq 0xffffffff810f0ba0 > clocksource_touch_watchdog > 0xffffffffa073e023 mdb_watchdogs+0x13: callq 0xffffffff810dea20 > rcu_cpu_stall_reset > 0xffffffffa073e028 mdb_watchdogs+0x18: callq 0xffffffff811337c0 > touch_nmi_watchdog > 0xffffffffa073e02d mdb_watchdogs+0x1d: pop %rbp > 0xffffffffa073e02e mdb_watchdogs+0x1e: data16 > 0xffffffffa073e030 mdb_watchdogs+0x20: retq > 0xffffffffa073e031 mdb_watchdogs+0x21: nopw %cs:0x0(%rax,%rax,1)) > mdb|mdb: > 0xffffffffa073e040 mdb: nopl 0x0(%rax,%rax,1)) << > 0xffffffffa073e045 mdb+0x5: push %rbp > 0xffffffffa073e046 mdb+0x6: mov %rsp,%rbp > 0xffffffffa073e049 mdb+0x9: push %r15 > 0xffffffffa073e04b mdb+0xb: push %r14 > 0xffffffffa073e04d mdb+0xd: mov %rdi,%r14 > 0xffffffffa073e050 mdb+0x10: push %r13 > (0)> u mdb_watchdogs > mdb|mdb_watchdogs: > 0xffffffffa073e010 0F1F440000 nop DWORD PTR [rax+rax]=0x0 << > 0xffffffffa073e015 55 push rbp > 0xffffffffa073e016 4889E5 mov rbp,rsp > 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 > here are some examples from in-tree code: (2)> u async_schedule async_schedule: 0xffffffff810a0210 0F1F440000 nop DWORD PTR [rax+rax]=0x0 0xffffffff810a0215 55 push rbp 0xffffffff810a0216 48C7C2E0D79F81 mov rdx,0xffffffff819fd7e0 0xffffffff810a021d 4889E5 mov rbp,rsp 0xffffffff810a0220 E86BFEFFFF call __async_schedule 0xffffffff810a0225 5D pop rbp 0xffffffff810a0226 C3 ret 0xffffffff810a0227 660F1F840000000000 nop WORD PTR [rax+rax]=0x0000 async_schedule_domain: 0xffffffff810a0230 0F1F440000 nop DWORD PTR [rax+rax]=0x0 0xffffffff810a0235 55 push rbp 0xffffffff810a0236 4889E5 mov rbp,rsp 0xffffffff810a0239 E852FEFFFF call __async_schedule 0xffffffff810a023e 5D pop rbp 0xffffffff810a023f C3 ret current_is_async: 0xffffffff810a0240 0F1F440000 nop DWORD PTR [rax+rax]=0x0 0xffffffff810a0245 65488B3C2540BD0000 mov rdi,gs:0xbd40 0xffffffff810a024e F6471420 test BYTE PTR [rdi+20]=0x00,0x20 (2)> u schedule schedule: 0xffffffff8167c280 0F1F440000 nop DWORD PTR [rax+rax]=0x0 0xffffffff8167c285 55 push rbp 0xffffffff8167c286 65488B042540BD0000 mov rax,gs:0xbd40 0xffffffff8167c28f 4889E5 mov rbp,rsp 0xffffffff8167c292 53 push rbx 0xffffffff8167c293 488B10 mov rdx,QWORD PTR [rax]=0x0 0xffffffff8167c296 4885D2 test rdx,rdx 0xffffffff8167c299 740A je schedule+0x25 (0xffffffff8167c2a5) (down) 0xffffffff8167c29b 4883B8080A000000 cmp QWORD PTR [rax+0xa08]=0x0,0x0 0xffffffff8167c2a3 741E je schedule+0x43 (0xffffffff8167c2c3) (down) 0xffffffff8167c2a5 65488B1C2584410100 mov rbx,gs:0x14184 0xffffffff8167c2ae 31FF xor edi,edi 0xffffffff8167c2b0 E8CBF8FFFF call __schedule 0xffffffff8167c2b5 488B8308C0FFFF mov rax,QWORD PTR [rbx-0x3ff8]=0x4B53414D5F534B41 0xffffffff8167c2bc A808 test al,0x8 0xffffffff8167c2be 75EE jne schedule+0x2e (0xffffffff8167c2ae) (up) 0xffffffff8167c2c0 5B pop rbx 0xffffffff8167c2c1 5D pop rbp 0xffffffff8167c2c2 C3 ret (2)> g Jeff
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