On Wed, Oct 10, 2018 at 3:10 PM, Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote: >>>>>>> Just flooding out of memory messages can trigger RCU stall problems. >>>>>>> For example, a severe skbuff_head_cache or kmalloc-512 leak bug is causing >>>>>> >>>>>> [...] >>>>>> >>>>>> Quite some of them, indeed! I guess we want to rate limit the output. >>>>>> What about the following? >>>>> >>>>> A bit unrelated, but while we are at it: >>>>> >>>>> I like it when we rate-limit printk-s that lookup the system. >>>>> But it seems that default rate-limit values are not always good enough, >>>>> DEFAULT_RATELIMIT_INTERVAL / DEFAULT_RATELIMIT_BURST can still be too >>>>> verbose. For instance, when we have a very slow IPMI emulated serial >>>>> console -- e.g. baud rate at 57600. DEFAULT_RATELIMIT_INTERVAL and >>>>> DEFAULT_RATELIMIT_BURST can add new OOM headers and backtraces faster >>>>> than we evict them. >>>>> >>>>> Does it sound reasonable enough to use larger than default rate-limits >>>>> for printk-s in OOM print-outs? OOM reports tend to be somewhat large >>>>> and the reported numbers are not always *very* unique. >>>>> >>>>> What do you think? >>>> >>>> I do not really care about the current inerval/burst values. This change >>>> should be done seprately and ideally with some numbers. >>> >>> I think Sergey meant that this place may need to use >>> larger-than-default values because it prints lots of output per >>> instance (whereas the default limit is more tuned for cases that print >>> just 1 line). > > Yes. The OOM killer tends to print a lot of messages (and I estimate that > mutex_trylock(&oom_lock) accelerates wasting more CPU consumption by > preemption). > >>> >>> I've found at least 1 place that uses DEFAULT_RATELIMIT_INTERVAL*10: >>> https://elixir.bootlin.com/linux/latest/source/fs/btrfs/extent-tree.c#L8365 >>> Probably we need something similar here. > > Since printk() is a significantly CPU consuming operation, I think that what > we need to guarantee is interval between the end of an OOM killer messages > and the beginning of next OOM killer messages is large enough. For example, > setup a timer with 5 seconds timeout upon the end of an OOM killer messages > and check whether the timer already fired upon the beginning of next OOM killer > messages. > >> >> >> In parallel with the kernel changes I've also made a change to >> syzkaller that (1) makes it not use oom_score_adj=-1000, this hard >> killing limit looks like quite risky thing, (2) increase memcg size >> beyond expected KASAN quarantine size: >> https://github.com/google/syzkaller/commit/adedaf77a18f3d03d695723c86fc083c3551ff5b >> If this will stop the flow of hang/stall reports, then we can just >> close all old reports as invalid. > > I don't think so. Only this report was different from others because printk() > in this report was from memcg OOM events without eligible tasks whereas printk() > in others are from global OOM events triggered by severe slab memory leak. Ack. I guess I just hoped deep down that we somehow magically get rid of all these reports with some simple change like this :)
