On 2019-08-15, Sebastian Andrzej Siewior [off-list ref] wrote:

quoted

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