Re: [PATCH RFC v2 tip/core/rcu 02/22] rcu: Add per-task state to RCU CPU stall warnings

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 18 Mar 2020 17:10:40 -0700
paulmck@kernel.org wrote:

> From: "Paul E. McKenney" <paulmck@kernel.org>
> 
> Currently, an RCU-preempt CPU stall warning simply lists the PIDs of
> those tasks holding up the current grace period.  This can be helpful,
> but more can be even more helpful.
> 
> To this end, this commit adds the nesting level, whether the task
> things it was preempted in its current RCU read-side critical section,

s/things/thinks/

> whether RCU core has asked this task for a quiescent state, whether the
> expedited-grace-period hint is set, and whether the task believes that
> it is on the blocked-tasks list (it must be, or it would not be printed,
> but if things are broken, best not to take too much for granted).
> 
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> ---
>  kernel/rcu/tree_stall.h | 38 ++++++++++++++++++++++++++++++++++++--
>  1 file changed, 36 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> index 502b4dd..e19487d 100644
> --- a/kernel/rcu/tree_stall.h
> +++ b/kernel/rcu/tree_stall.h
> @@ -192,14 +192,40 @@ static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
>  	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>  }
>  
> +// Communicate task state back to the RCU CPU stall warning request.
> +struct rcu_stall_chk_rdr {
> +	int nesting;
> +	union rcu_special rs;
> +	bool on_blkd_list;
> +};
> +
> +/*
> + * Report out the state of a not-running task that is stalling the
> + * current RCU grace period.
> + */
> +static bool check_slow_task(struct task_struct *t, void *arg)
> +{
> +	struct rcu_node *rnp;
> +	struct rcu_stall_chk_rdr *rscrp = arg;
> +
> +	if (task_curr(t))
> +		return false; // It is running, so decline to inspect it.

Since it can be locked on_rq(), should we report that too?

-- Steve

> +	rscrp->nesting = t->rcu_read_lock_nesting;
> +	rscrp->rs = t->rcu_read_unlock_special;
> +	rnp = t->rcu_blocked_node;
> +	rscrp->on_blkd_list = !list_empty(&t->rcu_node_entry);
> +	return true;
> +}
> +
>  /*
>   * Scan the current list of tasks blocked within RCU read-side critical
>   * sections, printing out the tid of each.
>   */
>  static int rcu_print_task_stall(struct rcu_node *rnp)
>  {
> -	struct task_struct *t;
>  	int ndetected = 0;
> +	struct rcu_stall_chk_rdr rscr;
> +	struct task_struct *t;
>  
>  	if (!rcu_preempt_blocked_readers_cgp(rnp))
>  		return 0;
> @@ -208,7 +234,15 @@ static int rcu_print_task_stall(struct rcu_node *rnp)
>  	t = list_entry(rnp->gp_tasks->prev,
>  		       struct task_struct, rcu_node_entry);
>  	list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
> -		pr_cont(" P%d", t->pid);
> +		if (!try_invoke_on_locked_down_task(t, check_slow_task, &rscr))
> +			pr_cont(" P%d", t->pid);
> +		else
> +			pr_cont(" P%d/%d:%c%c%c%c",
> +				t->pid, rscr.nesting,
> +				".b"[rscr.rs.b.blocked],
> +				".q"[rscr.rs.b.need_qs],
> +				".e"[rscr.rs.b.exp_hint],
> +				".l"[rscr.on_blkd_list]);
>  		ndetected++;
>  	}
>  	pr_cont("\n");