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

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Message-ID: <20180709085351.GC2476@hirez.programming.kicks-ass.net>
Date:   Mon, 9 Jul 2018 10:53:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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 10:11:50AM -0700, Paul E. McKenney wrote:
> 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.

This is only called from the !list_empty(rcu_tasks_holdout) loop in
rcu_tasks_kthread afaict, and that has a
schedule_timeout_interruptible(HZ) in it, which I read as once a second.

Which seems like an entirely reasonable amount of time to kick a task.
Not scheduling for a second is like an eternity.