[Bug 2064539] Re: Revert back frame pointers for ppc64el (remove -fno-omit-frame-pointer)

Frank Heimes 2064539 at bugs.launchpad.net
Mon Aug 26 12:14:54 UTC 2024 

Did the verification for the kernel (linux-generic) like above and based
on representative kernel modules (arch-specific and common):

noble:
$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 24.04 LTS
Release:	24.04
Codename:	noble
$ uname -a
Linux P10d-LPAR06 6.8.0-41-generic #41-Ubuntu SMP Fri Aug  2 21:00:36 UTC 2024 ppc64le ppc64le ppc64le GNU/Linux
$ sudo unzstd /usr/lib/debug/lib/modules/6.8.0-41-generic/kernel/arch/powerpc/crypto/aes-gcm-p10-crypto.ko.zst
/usr/lib/debug/lib/modules/6.8.0-41-generic/kernel/arch/powerpc/crypto/aes-gcm-p10-crypto.ko.zst: 338761 bytes 
$ readelf -wi /usr/lib/debug/lib/modules/6.8.0-41-generic/kernel/arch/powerpc/crypto/aes-gcm-p10-crypto.ko  | grep DW_AT_produce | grep -c mbackchain
0
$ readelf -wi /usr/lib/debug/lib/modules/6.8.0-41-generic/kernel/arch/powerpc/crypto/aes-gcm-p10-crypto.ko | grep DW_AT_produce | grep -c no-omit-frame-pointer
0
$ sudo unzstd /usr/lib/debug/lib/modules/6.8.0-41-generic/kernel/fs//nfs/nfsv4.ko.zst 
/usr/lib/debug/lib/modules/6.8.0-41-generic/kernel/fs//nfs/nfsv4.ko.zst: 18009201 bytes 
$ readelf -wi /usr/lib/debug/lib/modules/6.8.0-41-generic/kernel/fs//nfs/nfsv4.ko | grep DW_AT_produce | grep -c mbackchain
0
$ readelf -wi /usr/lib/debug/lib/modules/6.8.0-41-generic/kernel/fs//nfs/nfsv4.ko | grep DW_AT_produce | grep -c no-omit-frame-pointer
0

oracular:
$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu Oracular Oriole (development branch)
Release:	24.10
Codename:	oracular
$ uname -a
Linux P10d-LPAR06 6.11.0-4-generic #4-Ubuntu SMP Tue Aug 20 14:54:18 UTC 2024 ppc64le ppc64le ppc64le GNU/Linux
$ sudo unzstd /usr/lib/debug/lib/modules/6.11.0-4-generic/kernel/arch/powerpc/crypto/aes-gcm-p10-crypto.ko.zst
/usr/lib/debug/lib/modules/6.11.0-4-generic/kernel/arch/powerpc/crypto/aes-gcm-p10-crypto.ko.zst: 354129 bytes 
$ readelf -wi /usr/lib/debug/lib/modules/6.11.0-4-generic/kernel/arch/powerpc/crypto/aes-gcm-p10-crypto.ko | grep DW_AT_produce | grep -c mbackchain
0
$ readelf -wi /usr/lib/debug/lib/modules/6.11.0-4-generic/kernel/arch/powerpc/crypto/aes-gcm-p10-crypto.ko | grep DW_AT_produce | grep -c no-omit-frame-pointer
0
$ sudo unzstd /usr/lib/debug/lib/modules/6.11.0-4-generic/kernel/fs/nfs/nfsv4.ko.zst
/usr/lib/debug/lib/modules/6.11.0-4-generic/kernel/fs/nfs/nfsv4.ko.zst: 19618441 bytes 
$ readelf -wi /usr/lib/debug/lib/modules/6.11.0-4-generic/kernel/fs/nfs/nfsv4.ko | grep DW_AT_produce | grep -c mbackchain
0
$ readelf -wi /usr/lib/debug/lib/modules/6.11.0-4-generic/kernel/fs/nfs/nfsv4.ko | grep DW_AT_produce | grep -c no-omit-frame-pointer
0

(Btw. the negative test for mbackchain was done by intention, since this
needs to be set for s390x, but for s390x only. The changes were done at
the same time, verifying here, that is was really not set for ppc64el.)

Like one can see "-no-omit-frame-pointer" (and 'mbackchain') is not set,
hence successful verified.

Closing the affected 'linux (Ubuntu)' as 'Fix Released' - and with that
the project entry as well.

** Changed in: linux (Ubuntu Noble)
       Status: New => Fix Released

** Changed in: linux (Ubuntu Oracular)
       Status: New => Fix Released

** Changed in: ubuntu-power-systems
       Status: New => Fix Released

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https://bugs.launchpad.net/bugs/2064539

Title:
  Revert back frame pointers for ppc64el (remove -fno-omit-frame-
  pointer)

Status in The Ubuntu-power-systems project:
  Fix Released
Status in dpkg package in Ubuntu:
  Fix Released
Status in glibc package in Ubuntu:
  Fix Released
Status in linux package in Ubuntu:
  Fix Released
Status in dpkg source package in Noble:
  Fix Released
Status in glibc source package in Noble:
  Fix Released
Status in linux source package in Noble:
  Fix Released
Status in dpkg source package in Oracular:
  Fix Released
Status in glibc source package in Oracular:
  Fix Released
Status in linux source package in Oracular:
  Fix Released

Bug description:
  SRU Justification:

  [ Impact ]

   * Power's Linux ABIs all require an explicit call chain be stored on
  the call stack frames which are all accessible via the stack pointer.

   * Therefore, having a (soft/simulated) frame pointer does not improve
  backtraces at all on Power.

   * However, forcing a frame pointer via the -fno-omit-frame-pointer
  option negatively affects performance for multiple reasons: extra
  prologue/epilogue overhead and fewer shrink-wrapping opportunities.

   * Given -fno-omit-frame-pointer does not provide any improvements
  (backtraces or otherwise) and only reduces performance, -fno-omit-
  frame-pointers should not be used on Power.

   * So we are facing here a performance penalty without any gain - on
  this particular platform.

   * And sometimes (in rare cases like LP#2060108) frame pointers may
  even lead to failed builds.

  [ Test Plan ]

   * Due to the above description of the impact and rationale,
     this pragmatic approach for testing is given:

   * Build the affected packages where frame-pointers should be reverted
     using the updated dpkg package (that incl. the modified build defaults)
     on (or for) this particular platform.

   * Now frame-pointer usage be checked in the following different ways:

   * 1) For the ease of use (and thanks to Julian Klode), there is this python
        test script available that allows to verify a binary in regard to
        frame pointers:
        https://gist.github.com/julian-klode/85e55553f85c410a1b856a93dce77208
        https://gist.githubusercontent.com/julian-klode/85e55553f85c410a1b856a93dce77208/raw/488b8509e6f23fe48f917961fe711b285dcb2e28/dwprod.py
        requires python3-pyelftools

   * 2) Another more manual way is to verify based on debug symbols like this:
        - find and install the ddeb package
        - maybe extract the  file (e.g. unzstd)
        - use 'readelf -wi'
        - and grep for 'DW_AT_produce' (build options)
        - look for entries regarding frame-pointer
        The output may look similar to this:
        readelf -wi ./usr/lib/debug/lib/modules/6.8.0-38-generic/kernel/arch/s390/crypto/aes_s390.ko | grep DW_AT_produce
            <23>   DW_AT_producer    : (indirect string, offset: 0x7d): GNU AS 2.42
            <129>   DW_AT_producer    : (indirect string, offset: 0x3eef): GNU C11 13.2.0 -m64 -mpacked-stack -mbackchain -msoft-float -march=z13 -mtune=z16 -mindirect-branch=thunk-extern -mfunction-return=thunk-extern -mindirect-branch-table -mrecord-mcount -mnop-mcount -mfentry -mzarch -g -gdwarf-5 -O2 -std=gnu11 -p -fshort-wchar -funsigned-char -fno-common -fno-strict-aliasing -fno-asynchronous-unwind-tables -fno-delete-null-pointer-checks -fno-allow-store-data-races -fno-stack-protector -ftrivial-auto-var-init=zero -fno-stack-clash-protection -fzero-call-used-regs=used-gpr -fno-inline-functions-called-once -falign-functions=8 -fstrict-flex-arrays=3 -fno-strict-overflow -fstack-check=no -fconserve-stack -fsanitize=bounds-strict -fsanitize=shift -fsanitize=bool -fsanitize=enum -fPIC

   * 3) And maybe watching the build messages / log for the build options that
        were used (but that is probably not sufficient - it's better to inspect
        the output.)

  [ Where problems could occur ]

   * The dpkg modifications could have been done erroneously.
     A dpkg test build and/or builds of other packages with the modified dpkg
     version in place would show this.

   * The settings in dpkg might be overwritten by other settings/packages.
     Tests like above, would show this.

   * One may think there could be issues in an environment where some packages
     have frame-pointer enabled and other don't.
     This is fine and was confirmed by IBM toolchain team and ours
     (as well as by a longer running <weeks> test system,
      with FP disabled in kernel, that showed no issues - like expected).

  [ Other Info ]

   * These changes were implemented during the opening of the oracular series.
     The very same changes are backported to 24.04 LTS.

   * These only affect the ppc64el and s390x architectures,
     for other architectures it's a no-change upload.

   * We didn't see any fallout for these changes during the development
     on the oracular series, and therefore don't expect any fallout or
     regressions in 24.04 LTS either.

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