Re: [PATCH] mm,page_alloc: Serialize warn_alloc() if schedulable.

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 1 Jun 2017 15:28:08 +0200 Michal Hocko <mhocko@suse.com> wrote:

> On Thu 01-06-17 22:11:13, Tetsuo Handa wrote:
> > Michal Hocko wrote:
> > > On Thu 01-06-17 20:43:47, Tetsuo Handa wrote:
> > > > Cong Wang has reported a lockup when running LTP memcg_stress test [1].
> > >
> > > This seems to be on an old and not pristine kernel. Does it happen also
> > > on the vanilla up-to-date kernel?
> > 
> > 4.9 is not an old kernel! It might be close to the kernel version which
> > enterprise distributions would choose for their next long term supported
> > version.
> > 
> > And please stop saying "can you reproduce your problem with latest
> > linux-next (or at least latest linux)?" Not everybody can use the vanilla
> > up-to-date kernel!
> 
> The changelog mentioned that the source of stalls is not clear so this
> might be out-of-tree patches doing something wrong and dump_stack
> showing up just because it is called often. This wouldn't be the first
> time I have seen something like that. I am not really keen on adding
> heavy lifting for something that is not clearly debugged and based on
> hand waving and speculations.

I'm thinking we should serialize warn_alloc anyway, to prevent the
output from concurrent calls getting all jumbled together?

I'm not sure I buy the "this isn't a mainline kernel" thing. 
warn_alloc() obviously isn't very robust, but we'd prefer that it be
robust to peculiar situations, wild-n-wacky kernel patches, etc.  It's
a low-level thing and it should Just Work.

I do think ratelimiting will be OK - if the kernel is producing such a
vast stream of warn_alloc() output then nobody is going to be reading
it all anyway.  Probably just the first one is enough for operators to
understand what's going wrong.

So...  I think both.  ratelimit *and* serialize.  Perhaps a simple but
suitable way of doing that is simply to disallow concurrent warn_allocs:

	/* comment goes here */
	if (test_and_set_bit(0, &foo))
		return;
	...
	clear_bit(0, &foo);

or whatever?

(And if we do decide to go with "mm,page_alloc: Serialize warn_alloc()
if schedulable", please do add code comments explaining what's going on)

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