Messages in this thread | | | Date | Wed, 1 Aug 2018 19:46:10 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH] nohz: Fix missing tick reprog while interrupting inline timer softirq |
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On Wed, 1 Aug 2018, Frederic Weisbecker wrote: > 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())
Where does this happen? Why is anything in the idle loop doing a local_bh_disable/enable() pair?
Or are you talking about NOHZ FULL and arbitrary task context?
> 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.
I'm not really happy with that 'fix' because what happens if:
.... local_bh_enable() do_softirq() --> interrupt() tick_nohz_irq_exit(); arm_timer();
So if that new timer is the only one on the CPU, what is going to arm the timer hardware which was just switched off in tick_nohz_irq_exit()?
I haven't looked deep enough, but a simple unconditional call to tick_irq_exit() at the end of do_softirq() might do the trick.
Thanks,
tglx
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