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

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Oct 16, 2026 at 10:14:27AM +0530, K Prateek Nayak wrote:

> > @@ -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 ;-)

I would much rather do this than expose rq_lock().