• Frederic Weisbecker's avatar
    nohz: Fix missing tick reprogram when interrupting an inline softirq · 0a0e0829
    Frederic Weisbecker authored
    The full nohz tick is reprogrammed in irq_exit() only if the exit is not in
    a nesting interrupt. This stands as an optimization: whether a hardirq or a
    softirq is interrupted, the tick is going to be reprogrammed when necessary
    at the end of the inner interrupt, with even potential new updates on the
    timer queue.
    
    When soft interrupts are interrupted, it's assumed that they are executing
    on the tail of an interrupt return. In that case tick_nohz_irq_exit() is
    called after softirq processing to take care of the tick reprogramming.
    
    But the assumption is wrong: softirqs can be processed inline as well, ie:
    outside of an interrupt, like in a call to local_bh_enable() or from
    ksoftirqd.
    
    Inline softirqs don't reprogram the tick once they are done, as opposed to
    interrupt tail softirq processing. So if a tick interrupts an inline
    softirq processing, the next timer will neither be reprogrammed from the
    interrupting tick's irq_exit() nor after the interrupted softirq
    processing. This situation may leave the tick unprogrammed while timers are
    armed.
    
    To fix this, simply keep reprogramming the tick even if a softirq has been
    interrupted. That can be optimized further, but for now correctness is more
    important.
    
    Note that new timers enqueued in nohz_full mode after a softirq gets
    interrupted will still be handled just fine through self-IPIs triggered by
    the timer code.
    Reported-by: 's avatarAnna-Maria Gleixner <anna-maria@linutronix.de>
    Signed-off-by: 's avatarFrederic Weisbecker <frederic@kernel.org>
    Signed-off-by: 's avatarThomas Gleixner <tglx@linutronix.de>
    Tested-by: 's avatarAnna-Maria Gleixner <anna-maria@linutronix.de>
    Cc: stable@vger.kernel.org # 4.14+
    Link: https://lkml.kernel.org/r/1533303094-15855-1-git-send-email-frederic@kernel.org
    0a0e0829
softirq.c 18.5 KB