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

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Date:   Mon, 28 Nov 2016 02:58:09 -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 Mon, Nov 28, 2016 at 03:46:39PM +0900, Byungchul Park wrote:
> On Sat, Nov 26, 2016 at 04:00:46AM -0800, Paul E. McKenney wrote:
> > On Fri, Nov 25, 2016 at 12:34:26AM -0800, Paul E. McKenney wrote:
> > > 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!
> > 
> > And the patch shown below seems to do the trick.  Thoughts?
> 
> Hello,
> 
> I see.. I didn't check the log 'rcu: Stop treating in-kernel CPU-bound
> workloads as errors'.
> 
> If you are convinced to remove them, no objection.

It is quite possible that additional time-related adjustments are needed,
but this change is a step in the right direction.

							Thanx, Paul

> Thank you,
> Byungchul
> 
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > commit d2db185bfee894c573faebed93461e9938bdbb61
> > Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Date:   Fri Nov 25 00:07:23 2016 -0800
> > 
> >     rcu: Remove short-term CPU kicking
> >     
> >     Commit 4914950aaa12d ("rcu: Stop treating in-kernel CPU-bound workloads
> >     as errors") added a (relatively) short-timeout call to resched_cpu().
> >     This was inspired by as issue that was fixed by b7e7ade34e61 ("sched/core:
> >     Fix remote wakeups").  But given that this issue was fixed, it is time
> >     for the current commit to remove this call to resched_cpu().
> >     
> >     Reported-by: Byungchul Park <byungchul.park@....com>
> >     Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 5f4f80d07852..91a68e4e6671 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1345,12 +1345,6 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
> >  		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 + 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. */
> > -
> >  	return 0;
> >  }
> >  
>