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sched/isolation: Prevent boot crash when the boot CPU is nohz_full
Documentation/timers/no_hz.rst states that the "nohz_full=" mask must not include the boot CPU, which is no longer true after:08ae95f4fd
("nohz_full: Allow the boot CPU to be nohz_full"). However after:aae17ebb53
("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). Fixes:aae17ebb53
("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/
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2 changed files with 12 additions and 6 deletions
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@ -129,11 +129,8 @@ adaptive-tick CPUs: At least one non-adaptive-tick CPU must remain
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online to handle timekeeping tasks in order to ensure that system
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calls like gettimeofday() returns accurate values on adaptive-tick CPUs.
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(This is not an issue for CONFIG_NO_HZ_IDLE=y because there are no running
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user processes to observe slight drifts in clock rate.) Therefore, the
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boot CPU is prohibited from entering adaptive-ticks mode. Specifying a
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"nohz_full=" mask that includes the boot CPU will result in a boot-time
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error message, and the boot CPU will be removed from the mask. Note that
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this means that your system must have at least two CPUs in order for
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user processes to observe slight drifts in clock rate.) Note that this
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means that your system must have at least two CPUs in order for
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CONFIG_NO_HZ_FULL=y to do anything for you.
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Finally, adaptive-ticks CPUs must have their RCU callbacks offloaded.
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@ -46,7 +46,16 @@ int housekeeping_any_cpu(enum hk_type type)
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if (cpu < nr_cpu_ids)
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return cpu;
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return cpumask_any_and(housekeeping.cpumasks[type], cpu_online_mask);
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cpu = cpumask_any_and(housekeeping.cpumasks[type], cpu_online_mask);
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if (likely(cpu < nr_cpu_ids))
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return cpu;
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/*
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* Unless we have another problem this can only happen
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* at boot time before start_secondary() brings the 1st
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* housekeeping CPU up.
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*/
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WARN_ON_ONCE(system_state == SYSTEM_RUNNING ||
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type != HK_TYPE_TIMER);
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}
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}
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return smp_processor_id();
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