Re: [PATCH] printk: do cond_resched() between lines while outputting to consoles

[Tejun Heo <tj@kernel.org>]

Hello, Jan.

On Wed, Nov 25, 2015 at 10:05:22AM +0100, Jan Kara wrote:
> So did you particularly have an issue during console registration? Because

Yeap, we're seeing a small ratio of machines falling head over hills
during IPMI serial console registration.  Pumping out the messages
collected prior to registration takes too long triggering softlockup
warning on all forty something CPUs which pile a metric ton of
messages atop.  From then on, softlockup / rcu stall warnings repeat
themselves.  Some machines recover after >10mins of doing that.  The
log is hillarious to look at afterward.

> at least our customers mostly have issues during heavy use of ordinary
> printk (e.g. during boot or when hardware gets probed) and your change
> doesn't affect that case. That being said if you really hit a case where

Hah, that must be a lot of messages being printk'd.

> your patch helps, then I have no problem with it (you can add my Acked-by).
> 
> At Kernel Summit I spoke with Linus and Andrew regarding printk softlockups
> and we ended up with a decision that we decouple queueing into kernel
> ringbuffer from the actual printing into console which would happen from
> kthread / workqueue. Then the lockups would be solved by printing to
> console happening from schedulable context and printk() as such being
> independent from console speed. We only have to have some special cases
> there for crashes so that messages get printed synchronously in that case.

Yeah, we'd prolly want to make the behavior contingent on the time
taken and so on.  At any rate, even with workqueue-deferred dumping,
this patch would still be necessary for non-preemptible kernels;
otherwise, there's no cond_resched() in printing path right now.

Thanks.

-- 
tejun