linux-kernel - Re: [RFC] Make need_resched() return true when rcu_urgent

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Message-Id: <20180706171150.GI3593@linux.vnet.ibm.com>
Date:   Fri, 6 Jul 2018 10:11:50 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     David Woodhouse <dwmw2@...radead.org>, mhillenb@...zon.de,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs
 requested

On Fri, Jul 06, 2018 at 06:29:05PM +0200, Peter Zijlstra wrote:
> On Fri, Jul 06, 2018 at 03:53:30PM +0100, David Woodhouse wrote:
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index e4d4e60..89f5814 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -1616,7 +1616,8 @@ static inline int spin_needbreak(spinlock_t *lock)
> >  
> >  static __always_inline bool need_resched(void)
> >  {
> > -	return unlikely(tif_need_resched());
> > +	return unlikely(tif_need_resched()) ||
> > +		rcu_urgent_qs_requested();
> >  }
> 
> Instead of making need_resched() touch two cachelines, I think I would
> prefer adding resched_cpu() to rcu_request_urgent_qs_task().

I used to do something like this, but decided that whacking each holdout
CPU over the head ten times a second was a bit much.

> The preempt state is alread a bit complicated and shadowed in the
> preempt_count (on some architectures) adding additional bits to it like
> this is just asking for trouble.

How about a separate need_resched_rcu() that includes the extra cache
miss?  Or open-coding the rcu_urgent_qs_requested()?

						Thanx, Paul