linux-kernel - Re: [RFC][PATCHv4 0/7] printk: introduce printing kernel threads

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Date:   Fri, 30 Jun 2017 14:11:24 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Jan Kara <jack@...e.cz>,
        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][PATCHv4 0/7] printk: introduce printing kernel threads

On Thu 2017-06-29 16:56:30, Sergey Senozhatsky wrote:
> yeah, I agree and understand that per-CPU printk kthreads
> is a bit... too much.
> 
> it was just a quick idea and I just gave it a try. but there
> are some potential takeaways from the series (not sure if you
> looked at the last patches of the series).
> 
> with per-CPU kthreads we easily
> 
> 1) can be quite flexible
>    there can be only certain CPUs that will take over printing duty.
>    so if you system has, say, IRQ affinity set up or anything else
>    that is critical, you can forbid printk offloading to those CPUs.
>    so "important" CPUs will not print out more than atomic_limit chars,
>    the rest of the job will be handled by "less important" CPUs.
> 
>    * this can be achieved with a single printk kthread.
>      but still, that's something I haven't thought about before this
>      series.

Sounds interesting. Well, I would prefer to leave this for
further optimization. It is a completely new area and it might
open another can of worms.

> 2) can offload printing to other CPUs from vprintk_emit()
>    and avoid any of scheduler->timekeeping->etc. paths. which will
>    replace printk_deferred().

I probably miss something. There is still called wake_up_process()
in this patchset and we could not do this in some scheduler and
timekeeping paths. Also we must not take the console_lock() in
these paths because it calls wake_up_process() as well. But
we want to flush few lines when possible.

Best Regards,
Petr