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    Subject[tip: sched/urgent] sched/isolation: {revent boot crash when the boot CPU is nohz_full
    The following commit has been merged into the sched/urgent branch of tip:

    Commit-ID: 8e3101b38dfc20848a23525b1e6e80bd1641d44c
    Gitweb: https://git.kernel.org/tip/8e3101b38dfc20848a23525b1e6e80bd1641d44c
    Author: Oleg Nesterov <oleg@redhat.com>
    AuthorDate: Thu, 11 Apr 2024 16:39:05 +02:00
    Committer: Thomas Gleixner <tglx@linutronix.de>
    CommitterDate: Wed, 24 Apr 2024 21:53:34 +02:00

    sched/isolation: {revent 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 commit 08ae95f4fd3b
    ("nohz_full: Allow the boot CPU to be nohz_full").

    However after commit 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).

    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>
    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/
    ---
    Documentation/timers/no_hz.rst | 7 ++-----
    kernel/sched/isolation.c | 11 ++++++++++-
    2 files changed, 12 insertions(+), 6 deletions(-)

    diff --git a/Documentation/timers/no_hz.rst b/Documentation/timers/no_hz.rst
    index f8786be..7fe8ef9 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.
    diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
    index 373d42c..2a262d3 100644
    --- a/kernel/sched/isolation.c
    +++ b/kernel/sched/isolation.c
    @@ -46,7 +46,16 @@ int housekeeping_any_cpu(enum hk_type type)
    if (cpu < nr_cpu_ids)
    return cpu;

    - return cpumask_any_and(housekeeping.cpumasks[type], cpu_online_mask);
    + cpu = cpumask_any_and(housekeeping.cpumasks[type], cpu_online_mask);
    + if (likely(cpu < nr_cpu_ids))
    + return cpu;
    + /*
    + * Unless we have another problem this can only happen
    + * at boot time before start_secondary() brings the 1st
    + * housekeeping CPU up.
    + */
    + WARN_ON_ONCE(system_state == SYSTEM_RUNNING ||
    + type != HK_TYPE_TIMER);
    }
    }
    return smp_processor_id();
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