Messages in this thread Patch in this message | | | From | Frederic Weisbecker <> | Subject | [PATCH] nohz: Fix missing tick reprog while interrupting inline timer softirq | Date | Wed, 1 Aug 2018 00:52:50 +0200 |
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Before updating the full nohz tick or the idle time on IRQ exit, we check first if we are not in a nesting interrupt, whether the inner interrupt is a hard or a soft IRQ.
There is a historical reason for that: the dyntick idle mode used to reprogram the tick on IRQ exit, after softirq processing, and there was no point in doing that job in the outer nesting interrupt because the tick update will be performed through the end of the inner interrupt eventually, with even potential new timer updates.
One corner case could show up though: if an idle tick interrupts a softirq executing inline in the idle loop (through a call to local_bh_enable()) after we entered in dynticks mode, the IRQ won't reprogram the tick because it assumes the softirq executes on an inner IRQ-tail. As a result we might put the CPU in sleep mode with the tick completely stopped whereas a timer can still be enqueued. Indeed there is no tick reprogramming in local_bh_enable(). We probably asssumed there was no bh disabled section in idle, although there didn't seem to be debug code ensuring that.
Nowadays the nesting interrupt optimization still stands but only concern full dynticks. The tick is stopped on IRQ exit in full dynticks mode and we want to wait for the end of the inner IRQ to reprogramm the tick. But in_interrupt() doesn't make a difference between softirqs executing on IRQ tail and those executing inline. What was to be considered a corner case in dynticks-idle mode now becomes a serious opportunity for a bug in full dynticks mode: if a tick interrupts a task executing softirq inline, the tick reprogramming will be ignored and we may exit to userspace after local_bh_enable() with an enqueued timer that will never fire.
To fix this, simply keep reprogramming the tick if we are in a hardirq interrupting softirq. We can still figure out a way later to restore this optimization while excluding inline softirq processing.
Reported-by: Anna-Maria Gleixner <anna-maria@linutronix.de> Signed-off-by: Frederic Weisbecker <frederic@kernel.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@kernel.org> --- kernel/softirq.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/softirq.c b/kernel/softirq.c index 900dcfe..0980a81 100644 --- a/kernel/softirq.c +++ b/kernel/softirq.c @@ -386,7 +386,7 @@ static inline void tick_irq_exit(void) /* Make sure that timer wheel updates are propagated */ if ((idle_cpu(cpu) && !need_resched()) || tick_nohz_full_cpu(cpu)) { - if (!in_interrupt()) + if (!in_irq()) tick_nohz_irq_exit(); } #endif -- 2.7.4
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