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

Steven Rostedt
2020-03-19T17:27:36Z

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");