Re: [PATCH tip/core/rcu 07/15] rcu: Add event tracing to ->gp_tasks update at GP start

[Steven Rostedt <rostedt@goodmis.org>]

On Mon, 24 Jul 2017 14:44:36 -0700
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> There is currently event tracing to track when a task is preempted
> within a preemptible RCU read-side critical section, and also when that
> task subsequently reaches its outermost rcu_read_unlock(), but none
> indicating when a new grace period starts when that grace period must
> wait on pre-existing readers that have been been preempted at least once
> since the beginning of their current RCU read-side critical sections.
> 
> This commit therefore adds an event trace at grace-period start in
> the case where there are such readers.  Note that only the first
> reader in the list is traced.
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
>  kernel/rcu/tree_plugin.h | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 14ba496a13cd..3e3f92e981a1 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -636,10 +636,17 @@ static int rcu_print_task_exp_stall(struct rcu_node *rnp)
>   */
>  static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
>  {
> +	struct task_struct *t;
> +
>  	RCU_LOCKDEP_WARN(preemptible(), "rcu_preempt_check_blocked_tasks() invoked with preemption enabled!!!\n");
>  	WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp));
> -	if (rcu_preempt_has_tasks(rnp))
> +	if (rcu_preempt_has_tasks(rnp)) {

The only function of this if block is to fill the content of the
trace event, correct?

What about doing:

	if (trace_rcu_unlock_preempted_task_enabled() &&
	    rcu_preempt_has_tasks(rnp)) {

instead? The trace_rcu_unlock_preempted_task_enabled() is a static
branch (aka jump_label), which would make the above a constant branch
when tracing is not enabled, and would keep this from adding any extra
overhead.

-- Steve

>  		rnp->gp_tasks = rnp->blkd_tasks.next;
> +		t = container_of(rnp->gp_tasks, struct task_struct,
> +				 rcu_node_entry);
> +		trace_rcu_unlock_preempted_task(TPS("rcu_preempt-GPS"),
> +						rnp->gpnum, t->pid);
> +	}
>  	WARN_ON_ONCE(rnp->qsmask);
>  }
>