Re: [PATCH] writeback: report a Tasks-RCU quiescent state per cgwb drain pass

[Paul E. McKenney]

On Wed, Sep 09, 2026 at 08:16:54AM -1000, Tejun Heo wrote:
> (cc'ing Paul)
> 
> On Wed, Sep 09, 2026 at 06:01:07PM +0000, Josef Bacik wrote:
> > cleanup_offline_cgwbs_workfn() drains a dying cgwb by calling
> > cleanup_offline_cgwb() until it returns false, with a cond_resched()
> > between passes.  On a CONFIG_PREEMPTION kernel that cond_resched() does
> > nothing: _cond_resched() is a plain "return 0", and under
> > PREEMPT_DYNAMIC the full and lazy modes disable it.  Since commit
> > 7dadeaa6e851 ("sched: Further restrict the preemption modes") those are
> > the only two models on the architectures with PREEMPT_LAZY support,
> > arm64 and x86 among them, so the drain loop never reports a Tasks-RCU
> > quiescent state.
> > 
> > A worker draining a cgwb with millions of attached inodes runs for
> > minutes.  On a 6.18 arm64 host in lazy mode the cgwb worker drained one
> > dying cgroup's writeback domain for over 11 minutes.  A BPF program
> > unlink (bpf_trampoline_unlink_prog -> bpf_trampoline_update ->
> > unregister_ftrace_direct -> ftrace_shutdown -> synchronize_rcu_tasks())
> > waited on that grace period while holding the trampoline mutex, 42
> > tasks queued behind it in D state, and the hung task detector fired at
> > 614 s and panicked the host.  Any BPF or ftrace detach during a long
> > drain inherits the drain's length.
> > 
> > Fix this by calling cond_resched_tasks_rcu_qs() so we do not stall out
> > anybody who calls sycnrhonize_rcu_tasks().  We put this in a do { } while
> > loop because if we have many small cgroups cleanup_offline_cgwb() will
> > return false and we will never call cond_resched_tasks_rcu_qs(), creating
> > the same problem.
> > 
> > Fixes: c22d70a162d3 ("writeback, cgroup: release dying cgwbs by switching attached inodes")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Link: https://lore.kernel.org/bpf/9d444098-7c03-4163-af12-bd0a79a51443@paulmck-laptop/
> > Assisted-by: LLM
> > Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
> 
> Acked-by: Tejun Heo <tj@xxxxxxxxxx>
> 
> > ---
> >  mm/backing-dev.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> > index cecbcf9060a6..18e999053bae 100644
> > --- a/mm/backing-dev.c
> > +++ b/mm/backing-dev.c
> > @@ -910,8 +910,9 @@ static void cleanup_offline_cgwbs_workfn(struct work_struct *work)
> >  			continue;
> >  
> >  		spin_unlock_irq(&cgwb_lock);
> > -		while (cleanup_offline_cgwb(wb))
> > -			cond_resched();
> > +		do {
> > +			cond_resched_tasks_rcu_qs();
> > +		} while (cleanup_offline_cgwb(wb));
> 
> The patch looks fine but this overall seems fragile. cond_resched() was
> already marking "stuff that can take too long" but we need to use
> cond_resched_tasks_rcu_qs() if it can take *really* long. There gotta be a
> way to make this more maintainable. If always doing tasks_rcu_qs from
> cond_resched() is too expensive, can it be be gated behind something cheaper
> e.g. some tick based test?

This is the business end of cond_resched_tasks_rcu_qs() in preemptible
kernels (in which cond_resched() is nothingness):

# define rcu_tasks_classic_qs(t, preempt)				\
	do {								\
		if (!(preempt) && READ_ONCE((t)->rcu_tasks_holdout))	\
			WRITE_ONCE((t)->rcu_tasks_holdout, false);	\
	} while (0)

This is pretty lightweight.  Adding a jiffies check would likely make
it more expensive.

Or am I missing your point?

							Thanx, Paul