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[Bug 1597419] Re: On __fget_light function issue


** Tags added: patch

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  On __fget_light function issue

Status in linux package in Ubuntu:

Bug description:
  Bellow is result of perf record:
  __fget_light  /proc/kcore                                                                                        
         │    ↓ je     30
         │      push   %rbp
         │      mov    %rsp,%rbp
         │      callq  0xffffffff8122a6a0
         │      test   %rax,%rax
         │    ↓ je     54
    0,83 │      or     $0x1,%rax
         │      pop    %rbp
         │    ← retq
         │30:   mov    0x20(%rax),%rax
    1,65 │      cmp    (%rax),%edi
    0,83 │    ↓ jae    51
    0,83 │      mov    0x8(%rax),%rax
    0,83 │      mov    %edi,%edi
         │      lea    (%rax,%rdi,8),%rax
    2,48 │      mov    (%rax),%rax
    2,48 │      test   %rax,%rax
         │    ↓ je     51
         │      test   %esi,0x44(%rax)
   70,25 │   ┌──jne    51
         │   │  repz   retq
         │51:└─→xor    %eax,%eax
         │    ← retq
         │54:   xor    %eax,%eax
         │      pop    %rbp
         │    ← retq
  and i attached one patch wich descrease from 70-100 cycles stall to 23 cycles stall on function.

  ProblemType: Bug
  DistroRelease: Ubuntu 16.04
  Package: linux-image-4.4.0-28-generic 4.4.0-28.47
  ProcVersionSignature: Ubuntu 4.4.0-28.47-generic 4.4.13
  Uname: Linux 4.4.0-28-generic x86_64
  ApportVersion: 2.20.1-0ubuntu2.1
  Architecture: amd64
   /dev/snd/controlC0:  asu        2742 F.... pulseaudio
  CurrentDesktop: MATE
  Date: Wed Jun 29 18:50:46 2016
  InstallationDate: Installed on 2016-04-29 (61 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-4.4.0-28-generic root=/dev/mapper/192--168--0--108--vg-root ro
   linux-restricted-modules-4.4.0-28-generic N/A
   linux-backports-modules-4.4.0-28-generic  N/A
   linux-firmware                            1.157.1
  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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