netdev - Re: [Patch net] net_sched: replace yield() with cond

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Date:   Thu, 06 Apr 2017 03:54:19 +0200
From:   Mike Galbraith <efault@....de>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [Patch net] net_sched: replace yield() with cond_resched()

On Wed, 2017-04-05 at 16:42 -0700, Cong Wang wrote:
> On Tue, Apr 4, 2017 at 10:56 PM, Mike Galbraith <efault@....de> wrote:
> > On Tue, 2017-04-04 at 22:19 -0700, Cong Wang wrote:
> > > On Tue, Apr 4, 2017 at 8:55 PM, Mike Galbraith <efault@....de> wrote:
> > 
> > > > That won't help, cond_resched() has the same impact upon a lone
> > > > SCHED_FIFO task as yield() does.. none.
> > > 
> > > Hmm? In the comment you quote:
> > > 
> > >  * 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().
> > > 
> > > So if cond_resched() doesn't help, why this misleading comment?
> > 
> > This is not an oh let's be nice guys thing, it's a perfect match of...
> > 
> > 
> >  * while (!event)
> >  *      yield();
> > (/copy/paste>
> > 
> > ..get off the CPU until this happens thing.  With nobody to yield the C
> > PU to, some_qdisc_is_busy() will remain true forever more.
> 
> 
> This is exactly the misleading part, a while-loop waiting for an event
> can always be a be-nice-for-others thing, because if not we can just
> spin as a spinlock.

Ah, but the kworker _is_ spinning on a 'lock' or sorts, starving the
'owner', ergo this polling loop fails the 'may be nice' litmus test. 
 No polling loop is safe without a guarantee that the polling thread
cannot block the loop breaking event.

	-Mike