Re: [RFC][PATCHv6 00/12] printk: introduce printing kernel thread

Sergey Senozhatsky
2017-12-19T01:25:02Z

On (12/18/17 20:08), Steven Rostedt wrote:
> > ... do you guys read my emails? which part of the traces I have provided
> > suggests that there is any improvement?
>
> The traces I've seen from you were from non-realistic scenarios.
> But I have hit issues with printk()s happening that cause one CPU to do all
> the work, where my patch would fix that. Those are the scenarios I'm
> talking about.

any hints about what makes your scenario more realistic than mine?
to begin with, what was the scenario?

[..]

> But I have hit issues with printk()s happening that cause one CPU to do all
> the work, where my patch would fix that. Those are the scenarios I'm
> talking about.

and this is exactly what I'm still observing. i_do_printks-1992 stops
printing, while console_sem is owned by another task. Since log_store()
much faster than call_console_drivers() AND console_sem owner is getting
preempted for unknown period of time, we end up having pending messages
in logbuf... and it's kworker/0:1-135 that prints them all.

systemd-udevd-671 [003] d..3 66.334866: offloading: set console_owner
kworker/0:1-135 [000] d..2 66.335999: offloading: vprintk_emit()->trylock FAIL will spin? :1
i_do_printks-1992 [002] d..2 66.345474: offloading: vprintk_emit()->trylock FAIL will spin? :0 x 1100
...
systemd-udevd-671 [003] d..3 66.345917: offloading: clear console_owner waiter != NULL :1

-ss