Re: [PATCH] rcu: Skip additional checks if rcu_cpu_stall_suppress is set

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Aug 08, 2017 at 10:50:26PM +0530, Neeraj Upadhyay wrote:
> If rcu_kick_kthreads is set, and gp is in progress, check_cpu_stall()
> does checks to figure out whether jiffies is past rsp->jiffies_stall,
> doing ordered accesses to avoid any false positives for new grace
> period initialization after a sufficiently large idle period. This
> extra processing can be skipped if rcu_cpu_stall_suppress is set.

Just to make sure I understand, the concern is that someone might have
booted with rcupdate.rcu_cpu_stall_suppress=1 (thus suppressing the RCU
CPU stall debugging warnings implemented later in check_cpu_stall()),
but later decided to also boot with rcutree.rcu_kick_kthreads=1 (thus
enabling kicking kthreads which check for RCU's grace-period kthreads
not being properly awakened)?

My immediate reaction is that if there is not much point in specifying
both rcutree.rcu_kick_kthreads=1 and rcupdate.rcu_cpu_stall_suppress=1.
But is there some use case that I am missing?

							Thanx, Paul

> Fixes: 8c7c4829a81c ("rcu: Awaken grace-period kthread if too long since FQS")
> Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
> ---
>  kernel/rcu/tree.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 51d4c3a..91b7552 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1562,10 +1562,13 @@ static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
>  	unsigned long js;
>  	struct rcu_node *rnp;
> 
> -	if ((rcu_cpu_stall_suppress && !rcu_kick_kthreads) ||
> -	    !rcu_gp_in_progress(rsp))
> +	if (!rcu_gp_in_progress(rsp))
>  		return;
>  	rcu_stall_kick_kthreads(rsp);
> +
> +	if (rcu_cpu_stall_suppress)
> +		return;
> +
>  	j = jiffies;
> 
>  	/*
> -- 
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