• Thomas Gleixner's avatar
    hrtimer: Reset hrtimer cpu base proper on CPU hotplug · d5421ea4
    Thomas Gleixner authored
    The hrtimer interrupt code contains a hang detection and mitigation
    mechanism, which prevents that a long delayed hrtimer interrupt causes a
    continous retriggering of interrupts which prevent the system from making
    progress. If a hang is detected then the timer hardware is programmed with
    a certain delay into the future and a flag is set in the hrtimer cpu base
    which prevents newly enqueued timers from reprogramming the timer hardware
    prior to the chosen delay. The subsequent hrtimer interrupt after the delay
    clears the flag and resumes normal operation.
    If such a hang happens in the last hrtimer interrupt before a CPU is
    unplugged then the hang_detected flag is set and stays that way when the
    CPU is plugged in again. At that point the timer hardware is not armed and
    it cannot be armed because the hang_detected flag is still active, so
    nothing clears that flag. As a consequence the CPU does not receive hrtimer
    interrupts and no timers expire on that CPU which results in RCU stalls and
    other malfunctions.
    Clear the flag along with some other less critical members of the hrtimer
    cpu base to ensure starting from a clean state when a CPU is plugged in.
    Thanks to Paul, Sebastian and Anna-Maria for their help to get down to the
    root cause of that hard to reproduce heisenbug. Once understood it's
    trivial and certainly justifies a brown paperbag.
    Fixes: 41d2e494
     ("hrtimer: Tune hrtimer_interrupt hang logic")
    Reported-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
    Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Sebastian Sewior <bigeasy@linutronix.de>
    Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
    Cc: stable@vger.kernel.org
    Link: https://lkml.kernel.org/r/alpine.DEB.2.20.1801261447590.2067@nanos