On Wed, 24 Jan 2018 01:01:53 +0900 Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx> wrote: > 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. And throttling the printks would stop the lock up too. > > 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. Right, but now you just made printk safe unreliable to get information out, because you need to wait for a schedule to occur, and if there's issues, like a deadlock, that thread will never run. And you just lost you lockdep splat. > > > 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. I'll add more capitals next time ;-) > > > > 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. How so? If we drop them, then the stuck printk has nothing to print and will move forward. I say once you start dropping printks due to recursion, keep dropping them. For at least a second, to allow them to stop killing the machine. > > > > 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. So printk safe is only triggered when at the same context? If we can guarantee that printk safe is triggered only when its because a printk is happening at the same context (not because of an interrupt, but really at the same context, using my context check), then I'm fine with delaying them to a work queue. That is, if we have this: printk() console_lock() <interrupt> printk() add to log buffer <print irq printk too> console_unlock(); printk() console_lock() <console does a printk> put in printk safe buffer trigger work queue console_unlock() <work queue> flush safe buffer printk() Then I'm fine with that. I have to look at the latest code. If this is indeed what we have, then I admit I misunderstood the problem you want to solve. I only want recursive printks (those that are actually triggered by doing a printk) to be allowed to be delayed. Make sense? -- Steve -- 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>
