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

From	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Newsgroups	linux.kernel
Subject	Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread
Date	2017-06-01 11:30 +0200
Message-ID	<tNuAx-HW-9@gated-at.bofh.it> (permalink)
References	(2 earlier) <tFsPg-5e7-9@gated-at.bofh.it> <tMp9U-6ft-19@gated-at.bofh.it> <tMrOq-8a6-11@gated-at.bofh.it> <tN6oy-1N5-13@gated-at.bofh.it> <tNsIp-7Yb-13@gated-at.bofh.it>
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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

Thread

Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread Petr Mladek <pmladek@suse.com> - 2017-05-29 11:30 +0200
  Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread Jan Kara <jack@suse.cz> - 2017-05-29 14:20 +0200
    Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-05-31 09:40 +0200
      Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread Andreas Mohr <andi@lisas.de> - 2017-06-01 00:00 +0200
      Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-06-01 09:30 +0200
        Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-06-01 09:30 +0200
        Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-06-01 11:30 +0200
  Re: [RFC][PATCHv3 2/5] printk: introduce printing kernel thread Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-05-31 09:30 +0200

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