On Tue, 2 Jun 2015, Michal Hocko wrote:
OOM killer might be triggered externally via sysrq+f. This is supposed
to kill a task no matter what e.g. a task is selected even though there
is an OOM victim on the way to exit. This is a big hammer for an admin
to help to resolve a memory short condition when the system is not able
to cope with it on its own in a reasonable time frame (e.g. when the
system is trashing or the OOM killer cannot make sufficient progress).
The forced OOM killing is currently wired into out_of_memory()
call which is kind of ugly because generic out_of_memory path
has to deal with configuration settings and heuristics which
are completely irrelevant to the forced OOM killer (e.g.
sysctl_oom_kill_allocating_task or OOM killer prevention for already
dying tasks). Some of those will not apply to sysrq because the handler
runs from the worker context.
check_panic_on_oom on the other hand will work and that is kind of
unexpected because sysrq+f should be usable to kill a mem hog whether
the global OOM policy is to panic or not.
It also doesn't make much sense to panic the system when no task cannot
be killed because admin has a separate sysrq for that purpose.
Let's pull forced OOM killer code out into a separate function
(force_out_of_memory) which is really trivial now. Also extract the core
of oom_kill_process into __oom_kill_process which doesn't do any
OOM prevention heuristics.
As a bonus we can clearly state that this is a forced OOM killer in the
OOM message which is helpful to distinguish it from the regular OOM
killer.
I'm not sure what the benefit of this is, and it's adding more code.
Having multiple pathways and requirements, such as constrained_alloc(), to
oom kill a process isn't any clearer, in my opinion. It also isn't
intended to be optimized since the oom killer called from the page
allocator and from sysrq aren't fastpaths. To me, this seems like only a
source code level change and doesn't make anything more clear but rather
adds more code and obfuscates the entry path.
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