netdev - Re: net/sched: latent livelock in dev_deactivate

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Message-ID: <20170406102844.jr66tcf4qfmlj3wm@hirez.programming.kicks-ass.net>
Date:   Thu, 6 Apr 2017 12:28:44 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     Mike Galbraith <efault@....de>, netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: net/sched: latent livelock in dev_deactivate_many() due to
 yield() usage

On Wed, Apr 05, 2017 at 05:31:05PM -0700, Stephen Hemminger wrote:
> On Sun, 02 Apr 2017 06:28:41 +0200
> Mike Galbraith <efault@....de> wrote:
> 
> > Livelock can be triggered by setting kworkers to SCHED_FIFO, then
> > suspend/resume.. you come back from sleepy-land with a spinning
> > kworker.  For whatever reason, I can only do that with an enterprise
> > like config, my standard config refuses to play, but no matter, it's
> > "Typical broken usage".
> > 
> > (yield() should be rendered dead)
> 
> The kernel is not normally built to have kworkers run at SCHED_FIFO.
> The user has do some action to alter the process priorities.
> 
> I classify this as user error. We don't support killing kworker threads
> either.

PI can boost anybody to FIFO, all you need is to be holding a lock.