Re: Mapping from preempted task to running task on same CPU

[K Prateek Nayak]

Hello Paul,

On 10/15/2026 11:11 PM, Paul E. McKenney wrote:
> Hello!
> 
> One of the commits [1] that I used to chase down a recent bug [2] (and
> thank you again for the fix!) uses the scheduler-internal cpu_curr()
> macro to map from a given CPU to the task currently running on that CPU.
> In the commit, the actual operation is mapping from a stalled (AKA
> preempted but runnable) task to the task running on the corresponding CPU.
> This tasks's ->comm and ->pid are then printed in the RCU CPU stall
> warning console output.
> 
> One of the real-time folks voted in favor being able to use this (the
> group is CCed here), which suggests that I push this commit to mainline.
> Which means that I need to do something other than my crude hack of
> creating a rcu_cpu_curr() function in kernel/sched/core.c to get access
> to the scheduler-private cpu_curr() macro.
> 
> Please tell me what I should instead be doing (in the check_slow_task()
> function shown below) to get at this information.

[..snip..]

> @@ -327,6 +332,9 @@ static int check_slow_task(struct task_struct *t, void *arg)
>  	rscrp->on_blkd_list = !list_empty(&t->rcu_node_entry);
>  	rscrp->rcu_rdr_running = task_curr(t);
>  	rscrp->rcu_rdr_boosted = 0;
> +	rscrp->rcu_rdr_cpu = cpu = task_cpu(t);
> +	rscrp->rcu_rdr_cpu_offline = cpu_is_offline(cpu);
> +	rscrp->rcu_rdr_cpu_task = th = rcu_cpu_curr(cpu);

If you don't mind taking the overhead of rq_lock, you can use
cpu_curr_snapshot(). It should give you gives a rcu protected
task struct of cpu_curr() and ...

>  	if (rscrp->on_blkd_list) {
>  		rnp = READ_ONCE(t->rcu_blocked_node);
>  		raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
> @@ -352,6 +360,7 @@ static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
>  	int ndetected = 0;
>  	struct rcu_stall_chk_rdr rscr;
>  	struct task_struct *t;
> +	struct task_struct *tc;
>  	struct task_struct *ts[8];
>  
>  	lockdep_assert_irqs_disabled();
> @@ -372,17 +381,24 @@ static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
>  	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>  	while (i) {
>  		t = ts[--i];
> -		if (task_call_func(t, check_slow_task, &rscr))
> +		if (task_call_func(t, check_slow_task, &rscr)) {
>  			pr_cont(" P%d", t->pid);
> -		else
> -			pr_cont(" P%d/%d:%c%c%c%c%c%c",
> +		} else {
> +			tc = rscr.rcu_rdr_cpu_task;
> +			pr_cont(" P%d/%d:%c%c%c%c%c%c CPU %d%s %s (%d)%s",
>  				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],
>  				".R"[rscr.rcu_rdr_running],
> -				".B?"[rscr.rcu_rdr_boosted]);
> +				".B?"[rscr.rcu_rdr_boosted],
> +				rscr.rcu_rdr_cpu,
> +				rscr.rcu_rdr_cpu_offline ? "!" : "",
> +				tc ? tc->comm : "???",
> +				tc ? tc->pid : -1,
> +				tc ? (is_idle_task(tc) ? " (idle!)" : "") : " (no task)");
> +		}
>  		lockdep_assert_irqs_disabled();

... since you are IRQs disabled, it should be fine to use it here.

If you don't want to take the overhead of rq_lock() + barrier,
you can perhaps define a cpu_curr_snapshot_rcu() like:

    struct task_struct *cpu_curr_snapshot_rcu(int cpu)
    {
        return rcu_dereference_all(cpu_curr(cpu));
    }

in kernel/sched/core.c and expose it similar to
cpu_curr_snapshot().

I'll let Peter comment if he hates it or not ;-)

-- 
Thanks and Regards,
Prateek