On Wed, Aug 01, 2018 at 07:46:10PM +0200, Thomas Gleixner wrote: > 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?
It's about the idle loop. But I'm not aware of any example in practice, this is a purely theoretical, and more importantly it doesn't concern upstream anymore since we don't stop the tick from IRQ-tail anymore in dynticks-idle mode after Rafael's changes. > > > 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. In fact I should remove this whole paragraph, it's about code history that's not relevant anymore and it confuses the whole explanation which should concern nohz_full only. > > > > 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. Nope it should be ok, nohz_full is supposed to support timers queued on the fly while the tick is stopped, we issue a self-IPI if necessary: internal_add_timer() -> trigger_dyntick_cpu() -> wake_up_nohz_cpu() Thanks.