On 2019-08-15, Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> wrote: >> The attachment is here: >> https://bugzilla.kernel.org/attachment.cgi?id=284391 > > both traces have `printk' in it. Could you please explain what it is > doing? It should print something to the console. The printk code in > v4.19 vs later changed and now (by default) ERR levels are printed > immediately and will cause latency spikes. > > This would explain the spike but I don't understand how lazy preempt > is related here. Where in this trace is there a latency spike? I could not see one. Emergency messages are not involved here. You can see at 84239098us that systemd is writing to /dev/kmsg, but this is completed (haven only written to the ring buffer) by 84239118us (20us). printk schedules an irq_work to wake the printk thread. The irq_work begins at 84241028us. All it does is call wake_up_interruptible_all() on the queue. John Ogness
