Re: [PATCH] sched/fork: Fix timer_slack_ns inheritance for RT tasks

Re: [PATCH] sched/fork: Fix timer_slack_ns inheritance for RT tasks

[kernel test robot]

Hello,

kernel test robot noticed "ltp.prctl08.fail" on:

commit: 2aa2f0a955c7459279e861f62c770f5f81d64f4f ("[PATCH] sched/fork: Fix timer_slack_ns inheritance for RT tasks")
url: https://github.com/intel-lab-lkp/linux/commits/Xiao-Feng-via-B4-Relay/sched-fork-Fix-timer_slack_ns-inheritance-for-RT-tasks/20260520-210956
patch link: https://lore.kernel.org/all/20260520-sched-fork-fix-timer-slack-v1-1-d4fc937245b2@xiaomi.com/
patch subject: [PATCH] sched/fork: Fix timer_slack_ns inheritance for RT tasks

in testcase: ltp
version: 
with following parameters:

	disk: 1HDD
	fs: ext4
	test: syscalls-05



config: x86_64-rhel-9.4-ltp
compiler: gcc-14
test machine: 36 threads 1 sockets Intel(R) Core(TM) i9-10980XE CPU @ 3.00GHz (Cascade Lake) with 128G memory

(please refer to attached dmesg/kmsg for entire log/backtrace)



If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <oliver.sang@intel.com>
| Closes: https://lore.kernel.org/oe-lkp/202605281011.2ba77cc3-lkp@intel.com


2026-05-29 16:17:35 kirk -f syscalls-05 --tmp-dir /fs/sda1/tmpdir
Host information
	Hostname:   lkp-csl-d02
	Python:     3.13.5 (main, Jun 25 2025, 18:55:22) [GCC 14.2.0]
	Directory:  /fs/sda1/tmpdir/kirk.root/tmp0lg24sa6

Connecting to SUT: default

Suite: syscalls-05
──────────────────

...

^[[1;37mprctl02: ^[[0m^[[1;32mpass^[[0m | ^[[1;33mtainted^[[0m  (0.011s)
^[[1;37mprctl08: ^[[0m^[[1;31mfail^[[0m | ^[[1;33mtainted^[[0m  (0.011s)   <---
^[[1;37mpreadv203: ^[[0m^[[1;36mskip^[[0m | ^[[1;33mtainted^[[0m  (47.775s)
^[[1;37mprofil01: ^[[0m^[[1;32mpass^[[0m | ^[[1;33mtainted^[[0m  (5.005s)

...


Execution time: 9m 8s

Disconnecting from SUT: default

Target information
──────────────────
Kernel:   Linux 7.1.0-rc4+ #1 SMP PREEMPT_DYNAMIC Tue May 26 16:01:55 CST 2026
Cmdline:  ip=::::lkp-csl-d02::dhcp
          root=/dev/ram0
          RESULT_ROOT=/result/ltp/1HDD-ext4-syscalls-05/lkp-csl-d02/debian-13-x86_64-20250902.cgz/x86_64-rhel-9.4-ltp/gcc-14/2aa2f0a955c7459279e861f62c770f5f81d64f4f/0
          BOOT_IMAGE=/pkg/linux/x86_64-rhel-9.4-ltp/gcc-14/2aa2f0a955c7459279e861f62c770f5f81d64f4f/vmlinuz-7.1.0-rc4+
          branch=linux-devel/devel-hourly-20260521-050335
          job=/lkp/jobs/scheduled/lkp-csl-d02/ltp-1HDD-ext4-syscalls-05-debian-13-x86_64-20250902.cgz-2aa2f0a955c7-20260526-26280-5dbkeb-0.yaml
          user=lkp
          ARCH=x86_64
          kconfig=x86_64-rhel-9.4-ltp
          commit=2aa2f0a955c7459279e861f62c770f5f81d64f4f
          intremap=posted_msi
          acpi_rsdp=0x36983000
          max_uptime=14400
          LKP_SERVER=10.239.97.5
          nokaslr
          selinux=0
          debug
          apic=debug
          sysrq_always_enabled
          rcupdate.rcu_cpu_stall_timeout=100
          net.ifnames=0
          printk.devkmsg=on
          panic=-1
          softlockup_panic=1
          nmi_watchdog=panic
          oops=panic
          load_ramdisk=2
          prompt_ramdisk=0
          drbd.minor_count=8
          systemd.log_level=err
          ignore_loglevel
          console=tty0
          earlyprintk=ttyS0,115200
          console=ttyS0,115200
          vga=normal
          rw
          keep_initrds=/osimage/pkg/debian-13-x86_64-20250902.cgz/ltp-x86_64-8cd7644a5-1_20260514.cgz
          acpi_rsdp=0x36983000
Machine:  unknown
Arch:     x86_64
RAM:      114660756 kB
Swap:     0 kB
Distro:   debian 13

────────────────────────
      TEST SUMMARY
────────────────────────
Suite:   syscalls-05
Runtime: 9m 4s
Runs:    189

Results:
    Passed:   1293
    Failed:   3
    Broken:   0
    Skipped:  38
    Warnings: 0

^[[1;31mFailures:^[[0m
    • prctl08

Session stopped



The kernel config and materials to reproduce are available at:
https://download.01.org/0day-ci/archive/20260528/202605281011.2ba77cc3-lkp@intel.com



-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH] sched/fork: Fix timer_slack_ns inheritance for RT tasks

[K Prateek Nayak]

Hello Oliver,

Thank you for the test!

On 5/28/2026 10:20 AM, kernel test robot wrote:
> Suite: syscalls-05
> ──────────────────
> 
> ...
> 
> ^[[1;37mprctl02: ^[[0m^[[1;32mpass^[[0m | ^[[1;33mtainted^[[0m  (0.011s)
> ^[[1;37mprctl08: ^[[0m^[[1;31mfail^[[0m | ^[[1;33mtainted^[[0m  (0.011s)   <---

I think we are failing here
https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/syscalls/prctl/prctl08.c#L86

for:

    {check_inherit_timerslack, 70000, 70000, "Child process"}

This checks if the default timerslack is set to the Parent's timerslack
at the time of fork and prctl(PR_SET_TIMERSLACK, 0) in the child process
expects the timer slack to go back that inherited value.

With this patch, instead of inheriting it, we are resetting it at 50us
at the time of fork. Since this is documented in man page, I believe the
test is correct and this will indeed be a behavior change.

-- 
Thanks and Regards,
Prateek