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

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Date:   Wed, 28 Jun 2017 15:17:32 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Jan Kara <jack@...e.cz>,
        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
Subject: Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread

On Thu 2017-06-01 16:21:02, Sergey Senozhatsky wrote:
> 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?

I am not sure if this is worth the resources. It think that one
big win of workqueues was that it reduced the amount of running
per-CPU kthreads. There are systems with thousands of CPUs.

I am a bit afraid to use workqueues for flushing consoles.
It would be another dependency and another risk.

Otherwise, per-CPU kthreads/workqueues primary handle per-CPU
resources. But printk_kthread would handle consoles that
need to be serialized anyway. It sounds weird to have
per-CPU task just to increase the chance that it will
get scheduled.

Best Regards,
Petr