linux-kernel - Re: System freezes after OOM

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Date:	Fri, 15 Jul 2016 13:02:17 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Michal Hocko <mhocko@...nel.org>
cc:	Ondrej Kozina <okozina@...hat.com>,
	Jerome Marchand <jmarchan@...hat.com>,
	Stanislav Kozina <skozina@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, dm-devel@...hat.com
Subject: Re: System freezes after OOM



On Fri, 15 Jul 2016, Michal Hocko wrote:

> On Fri 15-07-16 08:11:22, Mikulas Patocka wrote:
> > 
> > The stacktraces showed that the kcryptd process was throttled when it 
> > tried to do mempool allocation. Mempool adds the __GFP_NORETRY flag to the 
> > allocation, but unfortunatelly, this flag doesn't prevent the allocator 
> > from throttling.
> 
> Yes and in fact it shouldn't prevent any throttling. The flag merely
> says that the allocation should give up rather than retry
> reclaim/compaction again and again.
> 
> > I say that the process doing mempool allocation shouldn't ever be 
> > throttled. Maybe add __GFP_NOTHROTTLE?
> 
> A specific gfp flag would be an option but we are slowly running out of
> bit space there and I am not yet convinced PF_LESS_THROTTLE is
> unsuitable.

PF_LESS_THROTTLE will make it throttle less, but it doesn't eliminate 
throttling entirely. So, maybe add PF_NO_THROTTLE? But PF_* flags are also 
almost exhausted.

> I might be missing something but exactly this is what happens in
> wait_iff_congested no? If the bdi doesn't see the congestion it wakes up
> the reclaim context even before the timeout. Or are we talking past each
> other?

OK, I see that there is wait queue in congestion_wait. I didn't notice it 
before.

Mikulas

> -- 
> Michal Hocko
> SUSE Labs
>