[Bug 1908307] Re: Please ship a glibc build for profiling in libc6-prof

[Launchpad Bug Tracker]

This bug was fixed in the package glibc - 2.31-0ubuntu9.2

---------------
glibc (2.31-0ubuntu9.2) focal; urgency=medium

  * Drop check preventing using float128 which breaks new icc (LP: #1895358)
  * Detect debconf consistently in libc6.preinst and do not crash if it is
    not used (LP: #1902955)
  * Ship libc variant compiled for profiling in libc6-prof (LP: #1908307)
  * elf: Add endianness markup to ld.so.cache (Closes: #731082) (LP: #1906250)

 -- Balint Reczey <rbalint at ubuntu.com>  Wed, 16 Dec 2020 12:04:55 +0100

** Changed in: glibc (Ubuntu Focal)
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/1908307

Title:
  Please ship a glibc build for profiling in libc6-prof

Status in glibc package in Ubuntu:
  Fix Committed
Status in glibc source package in Focal:
  Fix Released
Status in glibc source package in Groovy:
  Fix Committed

Bug description:
  [Impact]

  * The C library shipped in libc6 is built with GCC's default behavior
  of omitting the frame pointer that makes tracing and profiling harder.
  Building GLibc with -fno-omit-frame-pointer and shipping it in an
  optional libc6-prof binary package would help tracing and profiling
  efforts.

  [Test Case]

  * Install libc6-prof and start tracing in one shell:

    sudo bpftrace -e 't:syscalls:sys_enter_clock_nanosleep /comm == "sleep"/ {printf("%s\n", ustack);}'
    Attaching 1 probe...

          clock_nanosleep+94
          __nanosleep+23
          0x5652a2e6b827

  * Run the traced program in a different shell
   env LD_LIBRARY_PATH=/lib/libc6-prof/x86_64-linux-gnu sleep 1

  
  * Repeat it without using the libc6-prof library:


   ubuntu at ff-glibc:~$ sleep 1

   ubuntu at ff-glibc:~$ sudo bpftrace -e 't:syscalls:sys_enter_clock_nanosleep /comm == "sleep"/ {printf("%s\n", ustack); }'
   Attaching 1 probe...

          clock_nanosleep+84

  [ Where problems could occur ]

  * Glibc does not build for some architectures with the additional -fno-omit-frame-pointer option which is tracked in LP: #1898049.
  * The -fno-omit-frame-pointer option may sneak in to the regular build, this is convered in the test case.
  * Libc6-prof may be slower thanks to the -fno-omit-frame-pointer option, this is expected and this is the reason for shipping it in a separate package.

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