linux-kernel - Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread

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Message-ID: <20170601072102.GB3506@jagdpanzerIV.localdomain>
Date:   Thu, 1 Jun 2017 16:21:02 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Jan Kara <jack@...e.cz>, Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Eric Biederman <ebiederm@...ssion.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Pavel Machek <pavel@....cz>,
        Andreas Mohr <andi@...as.de>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread

On (05/31/17 16:30), Sergey Senozhatsky wrote:
> On (05/29/17 14:12), Jan Kara wrote:
> [..]
> > Actually I had something very similar in old versions of my patch set. And
> > it didn't work very well. The problem was that e.g. sometimes scheduler
> > decided that printk kthread should run on the same CPU as the process
> > currently doing printing and in such case printk kthread never took over
> > printing and the machine locked up due to heavy printing.
> 
> hm, interesting.

that's a tricky problem to deal with.



... so may be we can have per-CPU printk kthreads then

	static DEFINE_PER_CPU(struct task_struct *, printk_kthread);


SMP hotplug threads, to be precise, the same way as watchdog has it. and
then during offloading we can wake_up any printk_kthread that is knowingly
not from this-CPU, all of them, let them compete for the console_sem.

just a quick idea.

thoughts?

	-ss