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

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Message-ID: <20170601092009.GA553@jagdpanzerIV.localdomain>
Date:   Thu, 1 Jun 2017 18:20:09 +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 (06/01/17 16:21), 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?

and we, of course, can provide a user space knob to enforce cpumask
of printk_kthreads, if someone doesn't want to have too many printk
kthreads (e.g. a system with 200 CPUs). so we can wake_up printk_kthread
only on given CPUs.

so I hacked a quick and dirty version of printing offloading using smp
threads, ran some tests and it seems to work better then the version
with a single printk_kthread.

overall this looks like the right direction. to me. well, I may be
wrong.

	-ss