linux-kernel - RE: [RESEND PATCH 1/1] mm: vmstat: Add OOM victims count in vmstat counter

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Date:	Wed, 14 Oct 2015 15:04:57 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	PINTU KUMAR <pintu.k@...sung.com>
cc:	akpm@...ux-foundation.org, minchan@...nel.org, dave@...olabs.net,
	mhocko@...e.cz, koct9i@...il.com, hannes@...xchg.org,
	penguin-kernel@...ove.sakura.ne.jp, bywxiaobai@....com,
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	sasha.levin@...cle.com, cl@...ux.com, fengguang.wu@...el.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, cpgs@...sung.com,
	pintu_agarwal@...oo.com, pintu.ping@...il.com,
	vishnu.ps@...sung.com, rohit.kr@...sung.com, c.rajkumar@...sung.com
Subject: RE: [RESEND PATCH 1/1] mm: vmstat: Add OOM victims count in vmstat
 counter

On Wed, 14 Oct 2015, PINTU KUMAR wrote:

> For me it was very helpful during sluggish and long duration ageing tests.
> With this, I don't have to look into the logs manually.
> I just monitor this count in a script. 
> The moment I get nr_oom_victims > 1, I know that kernel OOM would have happened
> and I need to take the log dump.
> So, then I do: dmesg >> oom_logs.txt
> Or, even stop the tests for further tuning.
> 

I think eventfd(2) was created for that purpose, to avoid the constant 
polling that you would have to do to check nr_oom_victims and then take a 
snapshot.

> > I disagree with this one, because we can encounter oom kills due to
> > fragmentation rather than low memory conditions for high-order allocations.
> > The amount of free memory may be substantially higher than all zone
> > watermarks.
> > 
> AFAIK, kernel oom happens only for lower-order (PAGE_ALLOC_COSTLY_ORDER).
> For higher-order we get page allocation failure.
> 

Order-3 is included.  I've seen machines with _gigabytes_ of free memory 
in ZONE_NORMAL on a node and have an order-3 page allocation failure that 
called the oom killer.

> > We've long had a desire to have a better oom reporting mechanism rather than
> > just the kernel log.  It seems like you're feeling the same pain.  I think it
> would be
> > better to have an eventfd notifier for system oom conditions so we can track
> > kernel oom kills (and conditions) in userspace.  I have a patch for that, and
> it
> > works quite well when userspace is mlocked with a buffer in memory.
> > 
> Ok, this would be interesting.
> Can you point me to the patches?
> I will quickly check if it is useful for us.
> 

https://lwn.net/Articles/589404.  It's invasive and isn't upstream.  I 
would like to restructure that patchset to avoid the memcg trickery and 
allow for a root-only eventfd(2) notification through procfs on system 
oom.
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