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

[Josh Triplett]

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.
> 
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>

> ---
>  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
>