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Date: Wed, 25 Jan 2017 13:11:50 -0500
From: Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH v6] mm: Add memory allocation watchdog kernel thread.
Message-ID: <20170125181150.GA16398@cmpxchg.org>
References: <1478416501-10104-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp>
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To: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: mhocko@suse.cz, linux-mm@kvack.org, linux-kernel@vger.kernel.org

On Sun, Nov 06, 2016 at 04:15:01PM +0900, Tetsuo Handa wrote:
> +- Why need to use it?
> +
> +Currently, when something went wrong inside memory allocation request,
> +the system might stall without any kernel messages.
> +
> +Although there is khungtaskd kernel thread as an asynchronous monitoring
> +approach, khungtaskd kernel thread is not always helpful because memory
> +allocating tasks unlikely sleep in uninterruptible state for
> +/proc/sys/kernel/hung_task_timeout_secs seconds.
> +
> +Although there is warn_alloc() as a synchronous monitoring approach
> +which emits
> +
> +  "%s: page allocation stalls for %ums, order:%u, mode:%#x(%pGg)\n"
> +
> +line, warn_alloc() is not bullet proof because allocating tasks can get
> +stuck before calling warn_alloc() and/or allocating tasks are using
> +__GFP_NOWARN flag and/or such lines are suppressed by ratelimiting and/or
> +such lines are corrupted due to collisions.

I'm not fully convinced by this explanation. Do you have a real life
example where the warn_alloc() stall info is not enough? If yes, this
should be included here and in the changelog. If not, the extra code,
the task_struct overhead etc. don't seem justified.

__GFP_NOWARN shouldn't suppress stall warnings, IMO. It's for whether
the caller expects allocation failure and is prepared to handle it; an
allocation stalling out for 10s is an issue regardless of the callsite.

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