linux-kernel - Re: [PATCH] trace: Set oom_score_adj to maximum for ring buffer allocating process

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Date:	Thu, 26 May 2011 13:33:48 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Vaibhav Nagarnaik <vnagarnaik@...gle.com>
cc:	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Michael Rubin <mrubin@...gle.com>,
	David Sharp <dhsharp@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] trace: Set oom_score_adj to maximum for ring buffer
 allocating process

On Thu, 26 May 2011, Vaibhav Nagarnaik wrote:

> > Hmm, have you tried this in practice? Yes we may kill the "echo" command
> > but it doesn't stop the ring buffer from being allocated, and thus
> > killing the echo command may not be enough, and those critical processes
> > that you are trying to protect will be killed next.
> >
> 
> Yes I did try this and found that it works as we intend it to. When
> oom-killer is invoked, it picks the process which has lowest
> oom_score_adj and kills it or one of its children.

s/lowest/highest/

> When the process is
> getting killed, any memory allocation from it would be returned -ENOMEM,
> which gets handled in our allocation process and we free up previously
> allocated memory.
> 

Not sure that's true, this is allocating with kzalloc_node(GFP_KERNEL), 
correct?  If current is oom killed, it will have access to all memory 
reserves which will increase the liklihood that the allocation will 
succeed before handling the SIGKILL.

> This API is now being used in other parts of kernel too, where it knows
> that the allocation could cause OOM.
> 

What's wrong with using __GFP_NORETRY to avoid oom killing entirely and 
then failing the ring buffer memory allocation?  Seems like a better 
solution than relying on the oom killer, since there may be other threads 
with a max oom_score_adj as well that would appear in the tasklist first 
and get killed unnecessarily.  Is there some ring buffer code that can't 
handle failing allocations appropriately?
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