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Re: net/sched: latent livelock in dev_deactivate_many() due to yield() usage

From Peter Zijlstra <peterz@infradead.org>
Newsgroups linux.kernel
Subject Re: net/sched: latent livelock in dev_deactivate_many() due to yield() usage
Date 2017-04-06 12:30 +0200
Message-ID <ttcPV-7fO-39@gated-at.bofh.it> (permalink)
References <trFjj-40B-3@gated-at.bofh.it> <tsFhf-2yy-13@gated-at.bofh.it> <tsJNT-5yF-1@gated-at.bofh.it> <tsLG1-6OS-1@gated-at.bofh.it>
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On Tue, Apr 04, 2017 at 10:25:19PM -0700, Cong Wang wrote:
> On Tue, Apr 4, 2017 at 8:20 PM, Mike Galbraith <efault@gmx.de> wrote:
> > -               while (some_qdisc_is_busy(dev))
> > -                       yield();
> > +               swait_event_timeout(swait, !some_qdisc_is_busy(dev), 1);
> >  }
> 
> I don't see why this is an improvement even if I don't care about the
> hardcoded timeout for now... Why the scheduler can make a better
> decision with swait_event_timeout() than with cond_resched()?

cond_resched() might be a no-op.

and doing yield() will result in a priority inversion deadlock. Imagine
the task doing yield() being the top priority (fifo99) task in the
system. Then it will simply spin forever, not giving whatever task is
required to make your condition true time to run.

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Thread

net/sched: latent livelock in dev_deactivate_many() due to yield()  usage Mike Galbraith <efault@gmx.de> - 2017-04-02 06:30 +0200
  Re: net/sched: latent livelock in dev_deactivate_many() due to  yield() usage Cong Wang <xiyou.wangcong@gmail.com> - 2017-04-05 00:40 +0200
    Re: net/sched: latent livelock in dev_deactivate_many() due to  yield() usage Mike Galbraith <efault@gmx.de> - 2017-04-05 05:30 +0200
      Re: net/sched: latent livelock in dev_deactivate_many() due to  yield() usage Cong Wang <xiyou.wangcong@gmail.com> - 2017-04-05 07:30 +0200
        Re: net/sched: latent livelock in dev_deactivate_many() due to  yield() usage Mike Galbraith <efault@gmx.de> - 2017-04-05 08:20 +0200
          Re: net/sched: latent livelock in dev_deactivate_many() due to  yield() usage Cong Wang <xiyou.wangcong@gmail.com> - 2017-04-06 02:00 +0200
            Re: net/sched: latent livelock in dev_deactivate_many() due to  yield() usage Mike Galbraith <efault@gmx.de> - 2017-04-06 03:10 +0200
        Re: net/sched: latent livelock in dev_deactivate_many() due to  yield() usage Peter Zijlstra <peterz@infradead.org> - 2017-04-06 12:30 +0200
  Re: net/sched: latent livelock in dev_deactivate_many() due to  yield() usage Stephen Hemminger <stephen@networkplumber.org> - 2017-04-06 02:40 +0200
    Re: net/sched: latent livelock in dev_deactivate_many() due to  yield() usage Mike Galbraith <efault@gmx.de> - 2017-04-06 03:30 +0200
    Re: net/sched: latent livelock in dev_deactivate_many() due to  yield() usage Peter Zijlstra <peterz@infradead.org> - 2017-04-06 12:30 +0200
      Re: net/sched: latent livelock in dev_deactivate_many() due to  yield() usage Peter Zijlstra <peterz@infradead.org> - 2017-04-06 13:10 +0200

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