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

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Date:	Fri, 27 May 2011 16:22:17 -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 Fri, 27 May 2011, Vaibhav Nagarnaik wrote:

> The tracing ring buffer is allocated from kernel memory. While
> allocating the memory, if OOM happens, the allocating process might not
> be the one that gets killed, since the ring-buffer memory is not
> allocated as process memory. Thus random processes might get killed
> during the allocation.
> 
> This patch makes sure that the allocating process is considered the most
> likely oom-kill-able process while the allocating is going on. Thus if
> oom-killer is invoked because of ring-buffer allocation, it is easier
> for the ring buffer memory to be freed and save important system
> processes from being killed.
> 
> This patch also adds __GFP_NORETRY flag to the ring buffer allocation
> calls to make it fail more gracefully if the system will not be able to
> complete the allocation request.
> 
> Signed-off-by: Vaibhav Nagarnaik <vnagarnaik@...gle.com>

Still not sure this is what we want, I'm afraid.

I like the addition of __GFP_NORETRY, but I don't understand the use of 
test_set_oom_score_adj() here.  Why can't we use oom_killer_disable(), 
allocate with __GFP_NORETRY, and then do oom_killer_enable()?

This prevents other tasks from getting oom killed themselves if they have 
oom_score_adj of OOM_SCORE_ADJ_MAX and allows the write to fail with 
-ENOMEM rather then being oom killed out from under us.

So why is test_set_oom_score_adj() better?

The alternative would be to setup an oom notifier for the ring buffer and 
stop allocating prior to killing a task and return a size that was smaller 
than what the user requested.
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