On 2022-02-08 23:56:34 [-0800], Christoph Hellwig wrote: > On Tue, Feb 08, 2022 at 03:34:05PM +0100, Sebastian Andrzej Siewior wrote: > > __raise_softirq_irqoff() adds a bit to the pending sofirq mask and this > > is it. The softirq won't be handled in a deterministic way but randomly > > when an interrupt fires and handles the softirq in its irq_exit() routine or > > if something randomly checks and handles pending softirqs in the call > > chain before the CPU goes idle. > > > > Add a local_bh_disable/enable() around the IRQ-off section which will > > handle pending softirqs. > > And I still haven't seen any good explanation of why this is useful. You need to handle the pending softirqs. If you don't handle them immediately or in a deterministic say (like on IRQ exit) then they will be handled at a random point. If you don't handle them at all, the CPU will go idle and at least the NO_HZ will complain about pending softirqs (can_stop_idle_tick()). You could still argue that the CPU will go down and the there are latencies involved but… I want to avoid waking ksoftirqd for that since there is no need to wake it and the pending work can be done in-context, right away. Sebastian
