On 2019-03-07, Sergey Senozhatsky <sergey.senozhatsky.work@xxxxxxxxx> wrote: >>>> When the console is constantly printing messages, I wouldn't say >>>> that looks like a lock-up scenario. It looks like the system is >>>> busy printing critical information to the console (which it is). >>> >>> What if we have N tasks/CPUs calling printk() simultaneously? >> >> Then they take turns printing their messages to the console, spinning >> until they get their turn. This still is not and does not look like a >> lock-up. But I think you already know this, so I don't understand the >> reasoning behind asking the question. Maybe you could clarify what >> you are getting at. > > Sorry John, the reasoning is that I'm trying to understand > why this does not look like soft or hard lock-up or RCU stall > scenario. The reason is that you are seeing data being printed on the console. The watchdogs (soft, hard, rcu, nmi) are all touched with each emergency message. > The CPU which spins on prb_lock() can have preemption disabled and, > additionally, can have local IRQs disabled, or be under RCU read > side lock. If consoles are busy, then there are CPUs which printk() > data and keep prb_lock contended; prb_lock() does not seem to be > fair. What am I missing? You are correct. Making prb_lock fair might be something we want to look into. Perhaps also based on the loglevel of what needs to be printed. (For example, KERN_ALERT always wins over KERN_CRIT.) > You probably talk about the case when all > printing CPUs are in preemptible contexts (assumingly this is what > is happening in dm-integrity case) so they can spin on prb_lock(), > that's OK. The case I'm talking about is - what if we have the same > situation, but then one of the CPUs printk()-s from !preemptible. > Does this make sense? Yes, you are referring to a worst case. We could have local_irqs disabled on every CPU while every CPU is hit with an NMI and all those NMIs want to dump a load of messages. The rest of the system will be frozen until those NMI printers can finish. But that is still not a lock-up. At some point those printers should finish and eventually the system should be able to resume. John Ogness
