From: Oleg Nesterov oleg@redhat.com

sched/isolation: Prevent boot crash when the boot CPU is nohz_full

[ Upstream commit 5097cbcb38e6e0d2627c9dde1985e91d2c9f880e ]

Documentation/timers/no_hz.rst states that the "nohz_full=" 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").

However after:

aae17ebb53cd ("workqueue: Avoid using isolated cpus' timers on queue_delayed_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 == 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 crecklin@redhat.com Signed-off-by: Oleg Nesterov oleg@redhat.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Signed-off-by: Ingo Molnar mingo@kernel.org Reviewed-by: Phil Auld pauld@redhat.com Acked-by: Frederic Weisbecker frederic@kernel.org 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 Krishanth.Jagaduri@sony.com --- Hi,

Before kernel 6.9, Documentation/timers/no_hz.rst states that "nohz_full=" 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-d404921e6d7e@sony.... --- 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..16bda468423e88090c0dc467ca7a5c7f3fd2bf02 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 CPU 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=y because there are no running -user processes to observe slight drifts in clock rate.) Therefore, the -boot CPU is prohibited from entering adaptive-ticks mode. Specifying a -"nohz_full=" 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=y to do anything for you.

Finally, adaptive-ticks CPUs must have their RCU callbacks offloaded.

--- base-commit: 219d54332a09e8d8741c1e1982f5eae56099de85 change-id: 20250129-send-oss-20250129-3c42dcf463eb

Best regards,