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However after: aae17ebb53cd ("workqueue: Avoid using isolated cpus' timers on queue_dela= yed_work") the kernel will crash at boot time in this case; housekeeping_any_cpu() returns an invalid CPU number until smp_init() brings the first housekeeping CPU up. Change housekeeping_any_cpu() to check the result of cpumask_any_and() and return smp_processor_id() in this case. This is just the simple and backportable workaround which fixes the symptom, but smp_processor_id() at boot time should be safe at least for type =3D=3D HK_TYPE_TIMER, this more or less matches the tick_do_timer_boot= _cpu logic. There is no worry about cpu_down(); tick_nohz_cpu_down() will not allow to offline tick_do_timer_cpu (the 1st online housekeeping CPU). [ Apply only documentation changes as commit which causes boot crash when boot CPU is nohz_full is not backported to stable kernels - Krishanth ] Fixes: aae17ebb53cd ("workqueue: Avoid using isolated cpus' timers on queue= _delayed_work") Reported-by: Chris von Recklinghausen Signed-off-by: Oleg Nesterov Signed-off-by: Thomas Gleixner Signed-off-by: Ingo Molnar Reviewed-by: Phil Auld Acked-by: Frederic Weisbecker Link: https://lore.kernel.org/r/20240411143905.GA19288@redhat.com Closes: https://lore.kernel.org/all/20240402105847.GA24832@redhat.com/ Cc: stable@vger.kernel.org # 5.4+ Signed-off-by: Krishanth Jagaduri Acked-by: Ingo Molnar --- Hi, Before kernel 6.9, Documentation/timers/no_hz.rst states that "nohz_full=3D" mask must not include the boot CPU, which is no longer true after commit 08ae95f4fd3b ("nohz_full: Allow the boot CPU to be nohz_full"). When trying LTS kernels between 5.4 and 6.6, we noticed we could use boot CPU as nohz_full but the information in the document was misleading. This was fixed upstream by commit 5097cbcb38e6 ("sched/isolation: Prevent boot crash when the boot CPU is nohz_full"). While it fixes the document description, it also fixes issue introduced by another commit aae17ebb53cd ("workqueue: Avoid using isolated cpus' timers on queue_delayed_work"). It is unlikely that upstream commit as a whole will be backported to stable kernels which does not contain the commit that introduced the issue of boot crash when boot CPU is nohz_full. Could we fix only the document portion in stable kernels 5.4+ that mentions boot CPU cannot be nohz_full? --- Changes in v2: - Add original changelog and trailers to commit message. - Add backport note for why only document portion is modified. - Link to v1: https://lore.kernel.org/r/20250205-send-oss-20250129-v1-1-d40= 4921e6d7e@sony.com --- Documentation/timers/no_hz.rst | 7 ++----- 1 file changed, 2 insertions(+), 5 deletions(-) diff --git a/Documentation/timers/no_hz.rst b/Documentation/timers/no_hz.rst index 065db217cb04fc252bbf6a05991296e7f1d3a4c5..16bda468423e88090c0dc467ca7= a5c7f3fd2bf02 100644 --- a/Documentation/timers/no_hz.rst +++ b/Documentation/timers/no_hz.rst @@ -129,11 +129,8 @@ adaptive-tick CPUs: At least one non-adaptive-tick CP= U must remain online to handle timekeeping tasks in order to ensure that system calls like gettimeofday() returns accurate values on adaptive-tick CPUs. (This is not an issue for CONFIG_NO_HZ_IDLE=3Dy because there are no runni= ng -user processes to observe slight drifts in clock rate.) Therefore, the -boot CPU is prohibited from entering adaptive-ticks mode. Specifying a -"nohz_full=3D" mask that includes the boot CPU will result in a boot-time -error message, and the boot CPU will be removed from the mask. Note that -this means that your system must have at least two CPUs in order for +user processes to observe slight drifts in clock rate.) Note that this +means that your system must have at least two CPUs in order for CONFIG_NO_HZ_FULL=3Dy to do anything for you. =20 Finally, adaptive-ticks CPUs must have their RCU callbacks offloaded. --- base-commit: 219d54332a09e8d8741c1e1982f5eae56099de85 change-id: 20250129-send-oss-20250129-3c42dcf463eb Best regards, --=20 Krishanth Jagaduri