Re: [PATCH] mm,oom: Defer dump_tasks() output.

[Michal Hocko <mhocko@suse.com>]

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