linux-kernel - Re: [PATCH 2/2] rcu: Force resched_cpu when jiffies >= rcu_state.jiffies

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Date:   Fri, 25 Nov 2016 00:34:26 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Byungchul Park <byungchul.park@....com>
Cc:     josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] rcu: Force resched_cpu when jiffies >=
 rcu_state.jiffies_resched

On Tue, Nov 22, 2016 at 05:12:20PM +0900, Byungchul Park wrote:
> On Wed, Nov 09, 2016 at 03:32:15PM +0900, Byungchul Park wrote:
> > Currently rcu code forces CPU into scheduler when jiffies >=
> > rcu_state.gp_start + jiffies_till_sched_qs, via resched_cpu().
> > 
> > It would be better to force CPU into scheduler when jiffies >=
> > rcu_state.jiffies_resched, too.
> 
> Hello,
> 
> I think these two patches are necessary to call resched_cpu() even in
> case of jiffies >= rcu_state.jiffies_resched, too. Am I wrong?
> 
> It would be appriciated if you let me know if I was wrong.

My current thought is that both the "if" statement and the call to
resched_cpu() should be removed, but I am still testing and working
through the timing.

Either way, I do very much appreciate your having called my attention
to this code!

							Thanx, Paul

> Thank you,
> Byungchul
> 
> > 
> > Signed-off-by: Byungchul Park <byungchul.park@....com>
> > ---
> >  kernel/rcu/tree.c | 5 ++---
> >  1 file changed, 2 insertions(+), 3 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index d8e8859..287f468 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1217,11 +1217,10 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
> >  				   READ_ONCE(*rcrmp) + rdp->rsp->flavor_mask);
> >  		}
> >  		rdp->rsp->jiffies_resched += 5; /* Re-enable beating. */
> > -	}
> >  
> > -	/* And if it has been a really long time, kick the CPU as well. */
> > -	if (ULONG_CMP_GE(jiffies, rdp->rsp->gp_start + jiffies_till_sched_qs))
> > +		/* And if it has been a really long time, kick the CPU as well. */
> >  		resched_cpu(rdp->cpu);  /* Force CPU into scheduler. */
> > +	}
> >  
> >  	return 0;
> >  }
> > -- 
> > 1.9.1
>