Re: [syzbot] [mm?] INFO: rcu detected stall in khugepaged (3)

[Paul E. McKenney]

On Wed, Aug 05, 2026 at 11:03:11PM -0700, Andrew Morton wrote:
> On Wed, 5 Aug 2026 13:28:16 -0700 "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:
> 
> > > collapse_scan_file()'s main loop has
> > > 
> > > 		if (need_resched()) {
> > > 			xas_pause(&xas);
> > > 			cond_resched_rcu();
> > > 		}
> > > 
> > > but that won't help with the RCU stall detector(?).
> > > 
> > > I suggest that a suitable fix here would be to add the analogous
> > > 
> > > 	if (rcu_i_need_to_take_a_break()) {
> > > 		rcu_read_unlock();
> > > 		rcu_take_a_break())	
> > > 		rcu_read_lock();
> > > 	}
> > > 
> > > (iirc rcu_read_unlock() does an rcu run, so rcu_take_a_break() isn't
> > > needed here)
> > > 
> > > Paul, wdyt?
> > 
> > Let's see...
> > 
> > The console log says "rcu_preempt detected stalls on CPUs/tasks",
> > which means that cond_resched() is a no-op, but it also means that
> > the rcu_read_unlock() in cond_resched_rcu() will directly take care of
> > informing RCU of the pause.
> > 
> > But that is clearly not happening.  Why?
> > 
> > Well, we have this:
> > 
> > rcu: 	Tasks blocked on level-0 rcu_node (CPUs 0-1): P37/1:b..l
> > 
> > This means that the task whose RCU read-side critical section is blocking
> > the current RCU grace period isn't even running, and thus cannot invoke
> > cond_resched_rcu(), let alone the rcu_read_unlock() within that function.
> > So an RCU CPU stall warning is expected behavior.  Or at least it is not
> > in any way ruled out.
> > 
> > What we need is RCU priority boosting.
> 
> Do we?  I'm suggesting we need need_resched_rcu()!

Understood, and I initially agreed with you.  Except that I then
found that the poor preempted task isn't executing anything at all.
Which means that an added need_resched_rcu() cannot possibly help.

> > Except that the .config file
> > does not enable this.  Not only is there no CONFIG_RCU_BOOST=y, there
> > is also no CONFIG_RCU_EXPERT=y and no CONFIG_PREEMPT_RT=y.  But there
> > is CONFIG_RT_MUTEX=y and CONFIG_RCU_EXPERT=y.
> > 
> > Because we don't have RCU priority boosting, if the load on the system
> > is heavy enough to prevent our poor preempted RCU reader (PID 37) from
> > running, the grace period cannot end.
> > 
> > I am not sure why this task is saving its stack, but maybe that is normal
> > for this code path?
> > 
> > My bemusement aside, I recommend running this test either with
> > non-preemptible RCU (CONFIG_PREEMPT_LAZY=y these days) or enabling RCU
> > priority boosting (CONFIG_RCU_EXPERT=y and CONFIG_RCU_BOOST=y).
> > 
> > Maybe RCU_BOOST should no longer depend on RCU_EXPERT?  I would of
> > course need ot remove the prompt ("Enable RCU priority boosting") to
> > avoid annoying Linus.  Maybe as shown below.
> > 
> > Thoughts?
> 
> If I'm understanding correctly, this workload is busted with this
> config and the proposed fix is to alter the config?  Well, why are we
> permitting that config at all?

Agreed, and my proposal is in fact to remove that config.  This assumes
that CONFIG_RCU_BOOST is ready for prime time, and given that I have
been testing it for years and that CONFIG_PREEMPT_RT has been using it
for years, I am cautiously optimistic.

> Seems to me that a solution to permit this config to work is very
> simple.  Something like:
> 
> 	time_t start;
> 
> 	rcu_read_lock();
> 	start = current_time();
> 
> 	for (lots of work) {
> 		...
> 		if (need_resched_rcu(start)) {
> 			cond_resched_rcu();
> 			start = current_time();
> 	}
> 
> Where need_resched_rcu() tests to see if we're getting close to hitting
> the watchdog timeout.
> 
> No?

No.

Added code doesn't help a task that has been preempted for some seconds
and thus isn't executing any code at all.

The task has been preempted while within an RCU read-side critical
section.  The sequence of events is as follows:

	rcu_read_lock();
	// preempted for many tens of seconds.
	cond_resched_rcu(); // doesn't help because it is never executed.
	rcu_read_unlock();

Or am I missing your point?

							Thanx, Paul