On Tue 03-09-19 23:20:48, Tetsuo Handa wrote: > On 2019/09/02 15:06, Michal Hocko wrote: > > On Sat 31-08-19 10:03:18, Tetsuo Handa wrote: > >> On 2019/08/30 19:35, Michal Hocko wrote: > >>> On Fri 30-08-19 19:04:53, Tetsuo Handa wrote: > >>>> If /proc/sys/vm/oom_dump_tasks != 0, dump_header() can become very slow > >>>> because dump_tasks() synchronously reports all OOM victim candidates, and > >>>> as a result ratelimit test for dump_header() cannot work as expected. > >>>> > >>>> This patch defers dump_tasks() till oom_mutex is released. As a result of > >>>> this patch, the latency between out_of_memory() is called and SIGKILL is > >>>> sent (and the OOM reaper starts reclaiming memory) will be significantly > >>>> reduced. > >>> > >>> This is adding a lot of code for something that might be simply worked > >>> around by disabling dump_tasks. Unless there is a real world workload > >>> that suffers from the latency and depends on the eligible task list then > >>> I do not think this is mergeable. > >>> > >> > >> People had to use /proc/sys/vm/oom_dump_tasks == 0 (and give up obtaining some > >> clue) because they worried stalls caused by /proc/sys/vm/oom_dump_tasks != 0 > >> while they have to use /proc/sys/vm/panic_on_oom == 0 because they don't want the > >> down time caused by rebooting. > > > > The main qustion is whether disabling that information is actually > > causing any real problems. > > I can't interpret your question. > If there is no real problem with forcing /proc/sys/vm/oom_dump_tasks == 0, > you had better remove dump_tasks(). There are still people who might be interested to see the oom selection decision and check it. I argue that they might be in minority and making oom_dump_tasks 0 by _default_ might make sense. There will still be an option to enable that information. I have no problem posting such a patch as an RFC. > >> This patch avoids stalls (and gives them some clue). > >> This patch also helps mitigating __ratelimit(&oom_rs) == "always true" problem. > >> A straightforward improvement. > > > > This is a wrong approach to mitigate that problem. Ratelimiting doesn't > > really work for any operation that takes a longer time. Solving that > > problem sounds usef in a generic way. > > Even if printk() is able to become asynchronous, a problem that "a lot of > printk() messages might be pending inside the printk buffer when we have to > write emergency messages to consoles due to entering critical situation" will remain. > This patch prevents dump_tasks() messages (which can become e.g. 32000 lines) from > pending in the printk buffer. Sergey and Petr, any comments to add? > > There is no better solution than "printk() users are careful not to exhaust > the printk buffer". > > > > >> If there are objections we can't apply this change, reasons would be something > >> like "This change breaks existing userspace scripts that parse OOM messages". > > > > No, not really. There is another aspect of inclusion criterion - > > maintainability and code complexity. This patch doesn't help neither. > > > > This patch helps improving robustness. No this patch just shifts the problem around while adding a nontrivial code. -- Michal Hocko SUSE Labs
