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

From	Cong Wang <xiyou.wangcong@gmail.com>
Newsgroups	linux.kernel
Subject	Re: net/sched: latent livelock in dev_deactivate_many() due to yield() usage
Date	2017-04-05 00:40 +0200
Message-ID	<tsFhf-2yy-13@gated-at.bofh.it> (permalink)
References	<trFjj-40B-3@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On Sat, Apr 1, 2017 at 9:28 PM, Mike Galbraith <efault@gmx.de> wrote:
> Greetings network wizards,
>
> Quoting kernel/sched/core.c:
> /**
>  * yield - yield the current processor to other threads.
>  *
>  * Do not ever use this function, there's a 99% chance you're doing it wrong.
>  *
>  * The scheduler is at all times free to pick the calling task as the most
>  * eligible task to run, if removing the yield() call from your code breaks
>  * it, its already broken.
>  *
>  * Typical broken usage is:
>  *
>  * while (!event)
>  *      yield();
>  *
>  * where one assumes that yield() will let 'the other' process run that will
>  * make event true. If the current task is a SCHED_FIFO task that will never
>  * happen. Never use yield() as a progress guarantee!!
>  *
>  * If you want to use yield() to wait for something, use wait_event().
>  * If you want to use yield() to be 'nice' for others, use cond_resched().
>  * If you still want to use yield(), do not!
>  */
>
> 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)


Thanks for the report! Looks like a quick solution here is to replace
this yield() with cond_resched(), it is harder to really wait for
all qdisc's to transmit all packets.

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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