linux-kernel - Re: [PATCH v2 tip/core/rcu 07/18] rcu: Add long-term CPU kicking

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Message-Id: <20170123193445.GI28085@linux.vnet.ibm.com>
Date:   Mon, 23 Jan 2017 11:34:45 -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 v2 tip/core/rcu 07/18] rcu: Add long-term CPU kicking

On Sat, Jan 21, 2017 at 12:42:55PM -0800, Josh Triplett wrote:
> On Tue, Jan 17, 2017 at 06:53:46PM -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.
> 
> Just realized an issue with this commit message: you're referring to
> what now appears as patch 8 in the past tense as something already done,
> and with a commit ID that probably doesn't work anymore.  You need to
> rephrase this to describe how it leads to a *subsequent* change rather
> than fixing something already changed.
> 
> With the commit message fixed:
> Reviewed-by: Josh Triplett <josh@...htriplett.org>

Good point!  It now reads as follows:

------------------------------------------------------------------------
This commit prepares for the removal of short-term CPU kicking (in a
subsequent commit).  It does so by starting to invoke resched_cpu()
for each holdout at each force-quiescent-state interval that is more
than halfway through the stall-warning interval.
------------------------------------------------------------------------

Does that work?

							Thanx, Paul

> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> 
> 
> 
> > ---
> >  kernel/rcu/tree.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 83bf054e194e..0e61b62e3f4a 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1225,6 +1225,12 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
> >  			 rdp->rsp->gp_start + 2 * jiffies_till_sched_qs) ||
> >  	    ULONG_CMP_GE(jiffies, rdp->rsp->gp_start + jiffies_till_sched_qs))
> >  		resched_cpu(rdp->cpu);  /* Force CPU into scheduler. */
> > +	/*
> > +	 * 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
> > 
>