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

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  [PATCH tip/core/rcu 08/20] rcu: Add long-term CPU kicking "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-01-14 10:20 +0100
    Re: [PATCH tip/core/rcu 08/20] rcu: Add long-term CPU kicking Josh Triplett <josh@joshtriplett.org> - 2017-01-16 09:00 +0100

#1558888 — [PATCH tip/core/rcu 08/20] rcu: Add long-term CPU kicking

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@linux.vnet.ibm.com>
---
 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

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#1559502

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@linux.vnet.ibm.com>

Could you please reverse the order of patches 7 and 8, so that
unnecessary splats don't occur between the two?

For the patch itself:
Reviewed-by: Josh Triplett <josh@joshtriplett.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
> 

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