Hi, Move printk and (some of) MM people to the recipients list.
On (01/10/18 09:02), Tejun Heo wrote: [..] > The particular case that we've been seeing regularly in the fleet was > the following scenario. > > 1. Console is IPMI emulated serial console. Super slow. Also > netconsole is in use. > 2. System runs out of memory, OOM triggers. > 3. OOM handler is printing out OOM debug info. > 4. While trying to emit the messages for netconsole, the network stack > / driver tries to allocate memory and then fail, which in turn > triggers allocation failure or other warning messages. printk was > already flushing, so the messages are queued on the ring. > 5. OOM handler keeps flushing but 4 repeats and the queue is never > shrinking. Because OOM handler is trapped in printk flushing, it > never manages to free memory and no one else can enter OOM path > either, so the system is trapped in this state. Tejun, we have a theory [since there are no logs available] that what you are looking at is something as follows: console_unlock() { for (;;) { call_console_drivers() kmalloc()/etc /* netconsole, skb kmalloc(), for instance */ __alloc_pages_slowpath() warn_alloc() /* a bunch of printk() -> log_store() */ } } Now, warn_alloc() is rate limited to DEFAULT_RATELIMIT_INTERVAL / DEFAULT_RATELIMIT_BURST so net console driver can add 10 warn_alloc() reports every 5 seconds to the logbuf. You have a "super slow" IPMI console and net console. So for every logbuf entry we do: console_unlock() { for (;;) { call_console_drivers(msg) -> IPMI_write() call_console_drivers(msg) -> netconsole_write() -> skb kmalloc() -> warn_alloc() -> ratelimit } } IPMI_write() is very slow, as you have noted, so it consumes time printing messages, simultaneously warn_alloc() rate limit depends on time. *Probably*, slow IPMI_write() is unable to flush 10 warn_alloc() reports under 5 seconds, which gives net console a chance to add another 10 warn_alloc()-s, while the previous 10 warn_alloc()-s have not been flushed yet. It seems that DEFAULT_RATELIMIT_INTERVAL / DEFAULT_RATELIMIT_BURST warn_alloc() rate limit is too permissive for your setup. Can you confirm that the theory is actually correct? If it is correct, then can we simply tweak warn_alloc() rate limit? Say, make it x2 / x4 / etc. times less verbose? E.g. "up to 5 warn_alloc()-s every 10 seconds"? What do MM folks think? -ss