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[Bug 1591193] Re: Useless operation on proc registry


On terminal run sudo perf record -e cycles -ag and then perf report
iov_iter_init  /proc/kcore                                                                                       
       │    Disassembly of section load0:
       │    ffffffff813fbae0 <load0>:
       │      push   %rbp
       │      mov    %gs:0x14204,%rax
       │      cmpq   $0xffffffffffffffff,-0x3fe8(%rax)
100,00 │      mov    %rsp,%rbp
       │    ↓ je     2f
       │      mov    %esi,(%rdi)
       │      mov    %rdx,0x18(%rdi)
       │1d:   mov    %rcx,0x20(%rdi)
       │      movq   $0x0,0x8(%rdi)
       │      mov    %r8,0x10(%rdi)
       │      pop    %rbp
       │    ← retq
       │2f:   or     $0x2,%esi
       │      mov    %rdx,0x18(%rdi)
       │      mov    %esi,(%rdi)
       │    ↑ jmp    1d

Under function iov_iter_init loose 100 cycles!

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  Useless operation on proc registry

Status in linux package in Ubuntu:

Bug description:
  Exist one situation on 64bit linux-kernel on every fuction (i'fill angry) on perf exist
    16,67 │       mov    %rsp,%rbp   
  which spent 16 cycles.
  This move from offset pointer reg to base pointer register , looking here for more info 
   On 64 bit platform not need this.

  ProblemType: Bug
  DistroRelease: Ubuntu 16.04
  Package: linux-image-4.4.0-22-generic 4.4.0-22.40
  ProcVersionSignature: Ubuntu 4.4.0-22.40-generic 4.4.8
  Uname: Linux 4.4.0-22-generic x86_64
  ApportVersion: 2.20.1-0ubuntu2.1
  Architecture: amd64
   /dev/snd/pcmC0D0p:   asu        4579 F...m pulseaudio
   /dev/snd/controlC0:  asu        4579 F.... pulseaudio
  CurrentDesktop: Unity
  Date: Fri Jun 10 16:05:09 2016
  InstallationDate: Installed on 2016-04-29 (41 days ago)
  InstallationMedia: Ubuntu-Studio 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
   lo        no wireless extensions.
   enp1s0    no wireless extensions.
  MachineType: Olidata S.p.A. ALABAMA
  ProcFB: 0 inteldrmfb
  ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-4.4.0-22-generic root=/dev/mapper/192--168--0--108--vg-root ro
   linux-restricted-modules-4.4.0-22-generic N/A
   linux-backports-modules-4.4.0-22-generic  N/A
   linux-firmware                            1.157
  SourcePackage: linux
  UpgradeStatus: No upgrade log present (probably fresh install)
  dmi.bios.date: 07/08/2008
  dmi.bios.vendor: American Megatrends Inc.
  dmi.bios.version: S0101
  dmi.board.asset.tag: To Be Filled By O.E.M.
  dmi.board.name: To be filled by O.E.M.
  dmi.board.vendor: To be filled by O.E.M.
  dmi.board.version: To be filled by O.E.M.
  dmi.chassis.asset.tag: To Be Filled By O.E.M.
  dmi.chassis.type: 3
  dmi.chassis.vendor: To Be Filled By O.E.M.
  dmi.chassis.version: To Be Filled By O.E.M.
  dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvrS0101:bd07/08/2008:svnOlidataS.p.A.:pnALABAMA:pvrToBeFilledByO.E.M.:rvnTobefilledbyO.E.M.:rnTobefilledbyO.E.M.:rvrTobefilledbyO.E.M.:cvnToBeFilledByO.E.M.:ct3:cvrToBeFilledByO.E.M.:
  dmi.product.name: ALABAMA
  dmi.product.version: To Be Filled By O.E.M.
  dmi.sys.vendor: Olidata S.p.A.

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