linux-kernel - Re: [PATCH tip/core/rcu 08/20] rcu: Add long-term CPU kicking

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Message-Id: <20170117010704.GY5238@linux.vnet.ibm.com>
Date:   Mon, 16 Jan 2017 17:07:04 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Josh Triplett <josh@...htriplett.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
        fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH tip/core/rcu 08/20] rcu: Add long-term CPU kicking

On Sun, Jan 15, 2017 at 11:58:07PM -0800, Josh Triplett wrote:
> On Sat, Jan 14, 2017 at 01:13:09AM -0800, Paul E. McKenney wrote:
> > Commit d2db185bfee8 ("rcu: Remove short-term CPU kicking") removed
> > frequent calls to resched_cpu(), which means that the only time
> > resched_cpu() is invoked is after an RCU CPU stall warning.  Although
> > this is good from an avoid-IPIs perspective, we should try to break
> > things loose -before- splatting.  This commit therefore starts invoking
> > resched_cpu() for each holdout at each force-quiescent-state interval
> > that is more than halfway through the stall-warning interval.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> 
> Could you please reverse the order of patches 7 and 8, so that
> unnecessary splats don't occur between the two?

Done!

							Thanx, Paul

> For the patch itself:
> Reviewed-by: Josh Triplett <josh@...htriplett.org>
> 
> >  kernel/rcu/tree.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 3c0d6c129718..5a4aaad75e76 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1220,6 +1220,13 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
> >  		rdp->rsp->jiffies_resched += 5; /* Re-enable beating. */
> >  	}
> >  
> > +	/*
> > +	 * If more than halfway to RCU CPU stall-warning time, do
> > +	 * a resched_cpu() to try to loosen things up a bit.
> > +	 */
> > +	if (jiffies - rdp->rsp->gp_start > rcu_jiffies_till_stall_check() / 2)
> > +		resched_cpu(rdp->cpu);
> > +
> >  	return 0;
> >  }
> >  
> > -- 
> > 2.5.2
> > 
>