linux-kernel - Re: [RFC][PATCHv6 00/12] printk: introduce printing kernel thread

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Date:   Wed, 10 Jan 2018 07:08:30 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     tj@...nel.org, rostedt@...dmis.org
Cc:     pmladek@...e.com, sergey.senozhatsky@...il.com, jack@...e.cz,
        akpm@...ux-foundation.org, peterz@...radead.org, rjw@...ysocki.net,
        pavel@....cz, linux-kernel@...r.kernel.org,
        sergey.senozhatsky.work@...il.com
Subject: Re: [RFC][PATCHv6 00/12] printk: introduce printing kernel thread

Tejun Heo wrote:
> The code might suck but I think this does replicate what we've been
> seeing regularly in the fleet.  The console side is pretty slow - IPMI
> faithfully emulating serial console.  I don't know it's doing 115200
> or even slower.  Please consider something like the following.

Emulated serial consoles tend to be slow.

> 
> * The kernel isn't preemptible.  Machine runs out of memory, hits OOM
>   condition.  It starts printing OOM information.
> 
> * Netconsole tries to send out OOM messages and tries memory
>   allocation which fails which then prints allocation failed messages.
>   Because this happens while already printing, it just queues the
>   messages to the buffer.  This repeats.

What? Does netconsole need to allocate memory when sending? I assume it does not.

> 
> * We're still in the middle of OOM and hasn't killed nobody, so memory
>   keeps being short and the printk ring buffer is continuously getting
>   filled by the above.  Also, after a bit, RCU stall warnings kick in
>   too producing more messages.

And mutex_trylock(&oom_lock) keeps wasting CPU. :-(