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SubjectRe: [patch -mm 4/9 v2] oom: remove compulsory panic_on_oom mode
On Tue, 16 Feb 2010 01:02:28 -0800 (PST)
David Rientjes <rientjes@google.com> wrote:

> On Tue, 16 Feb 2010, KAMEZAWA Hiroyuki wrote:
>
> > > You don't understand that the behavior has changed ever since
> > > mempolicy-constrained oom conditions are now affected by a compulsory
> > > panic_on_oom mode, please see the patch description. It's absolutely
> > > insane for a single sysctl mode to panic the machine anytime a cpuset or
> > > mempolicy runs out of memory and is more prone to user error from setting
> > > it without fully understanding the ramifications than any use it will ever
> > > do. The kernel already provides a mechanism for doing this, OOM_DISABLE.
> > > if you want your cpuset or mempolicy to risk panicking the machine, set
> > > all tasks that share its mems or nodes, respectively, to OOM_DISABLE.
> > > This is no different from the memory controller being immune to such
> > > panic_on_oom conditions, stop believing that it is the only mechanism used
> > > in the kernel to do memory isolation.
> > >
> > You don't explain why "we _have to_ remove API which is used"
> >
>
> First, I'm not stating that we _have_ to remove anything, this is a patch
> proposal that is open for review.
>
> Second, I believe we _should_ remove panic_on_oom == 2 because it's no
> longer being used as it was documented: as we've increased the exposure of
> the oom killer (memory controller, pagefault ooms, now mempolicy tasklist
> scanning), we constantly have to re-evaluate the semantics of this option
> while a well-understood tunable with a long history, OOM_DISABLE, already
> does the equivalent. The downside of getting this wrong is that the
> machine panics when it shouldn't have because of an unintended consequence
> of the mode being enabled (a mempolicy ooms, for example, that was created
> by the user). When reconsidering its semantics, I'd personally opt on the
> safe side and make sure the machine doesn't panic unnecessarily and
> instead require users to use OOM_DISABLE for tasks they do not want to be
> oom killed.
>

Please don't. I had a chance to talk with customer support team and talked
about panic_on_oom briefly. I understood that panic_on_oom_alyways+kdump
is the strongest tool for investigating customer's OOM situtation and do
the best advice to them. panic_on_oom_always+kdump is the 100% information
as snapshot when oom-killer happens. Then, it's easy to investigate and
explain what is wront. They sometimes discover memory leak (by some prorietary
driver) or miss-configuration of the system (as using unnecessary bounce buffer.)

Then, please leave panic_on_oom=always.
Even with mempolicy or cpuset 's OOM, we need panic_on_oom=always option.
And yes, I'll add something similar to memcg. freeze_at_oom or something.

Thanks,
-Kame






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