On (01/23/18 10:41), Steven Rostedt wrote: [..] > We can have more. But if printk is causing printks, that's a major bug. > And work queues are not going to fix it, it will just spread out the > pain. Have it be 100 printks, it needs to be fixed if it is happening. > And having all printks just generate more printks is not helpful. Even > if we slow them down. They will still never end. Dropping the messages is not the solution either. The original bug report report was - this "locks up my kernel". That's it. That's all people asked us to solve. With WQ we don't lockup the kernel, because we flush printk_safe in preemptible context. And people are very much expected to fix the misbehaving consoles. But that should not be printk_safe problem. > A printk causing a printk is a special case, and we need to just show > enough to let the user know that its happening, and why printks are > being throttled. Yes, we may lose data, but if every printk that goes > out causes another printk, then there's going to be so much noise that > we wont know what other things went wrong. Honestly, if someone showed > me a report where the logs were filled with printks that caused > printks, I'd stop right there and tell them that needs to be fixed > before we do anything else. And if that recursion is happening because > of another problem, I don't want to see the recursion printks. I want > to see the printks that show what is causing the recursions. I'll re-read this one tomorrow. Not quite following it. > > The problem is - we flush printk_safe too soon and printing CPU ends up > > in a lockup - it log_store()-s new messages while it's printing the pending > > No, the problem is that printks are causing more printks. Yes that will > make flushing them soon more likely to lock up the system. But that's > not the problem. The problem is printks causing printks. Yes. And ignoring those printk()-s by simply dropping them does not fix the problem by any means. > > ones. It's fine to do so when CPU is in preemptible context. Really, we > > should not care in printk_safe as long as we don't lockup the kernel. The > > misbehaving console must be fixed. If CPU is not in preemptible context then > > we do lockup the kernel. Because we flush printk_safe regardless of the > > current CPU context. If we will flush printk_safe via WQ then we automatically > > And if we can throttle recursive printks, then we should be able to > stop that from happening. pintk_safe was designed to be recursive. It was never designed to be used to troubleshoot or debug consoles. But it was designed to be recursive - because that's the sort of the problems it was meant to handle: recursive printks that would otherwise deadlock us. That's why we have it in the first place. > > add this "OK! The CPU is preemptible, we can log_store(), it's totally OK, we > > will not lockup it up." thing. Yes, we fill up the logbuf with probably needed > > and appreciated or unneeded messages. But we should not care in printk_safe. > > We don't lockup the kernel... And the misbehaving console must be fixed. > > I agree. Good. > > I disagree with "If we are having issues with irq_work, we are going to have > > issues with a work queue". There is a tremendous difference between irq_work > > on that CPU and queue_work_on(smp_proessor_id()). One does not care about CPU > > context, the other one does. > > But switching to work queue does not address the underlining problem > that printks are causing more printks. The only way to address those problems is to fix the console. That's the only. But that's not what I'm doing with my proposal. I fix the lockup scenario, the only reported problem so far. Whilst also keeping printk_safe around. -ss -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
