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

Josh Triplett
2017-01-16T07:58:21Z

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
>