** Description changed: SRU Justification: [ Impact ] - * Power's Linux ABIs all require an explicit call chain be stored on + * 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 + * 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 + * 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 + * 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 + * 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 + * 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: + * 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. + * 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: + * 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 + * 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 + * 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.) + * 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 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. + * 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). + * 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. + * These changes were implemented during the opening of the oracular series. + The very same changes are backported to 24.04 LTS. - * 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. + * 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.
-- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/2064539 Title: Revert back frame pointers for ppc64el (remove -fno-omit-frame- pointer) To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu-power-systems/+bug/2064539/+subscriptions -- ubuntu-bugs mailing list [email protected] https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
