Re: [PATCH] mm/vmscan: report RCU-tasks quiescent states in shrink_lruvec()

["Paul E. McKenney" <paulmck@kernel.org>]

On Mon, Aug 10, 2026 at 09:43:03PM -0700, Shakeel Butt wrote:
> On Mon, Aug 10, 2026 at 02:57:36AM -0700, Breno Leitao wrote:
> > I am seeing some rcu_tasks stalls in the Meta fleet during reclaim.
> > 
> >   INFO: rcu_tasks detected stalls on tasks:
> > 	0000000088620d09: .. nvcsw: 6735/6735 holdout: 1 idle_cpu: -1/8
> > 	task:GlobalCPUThread state:R  running task  pid:2552016 tgid:2524552
> >   Call Trace:
> >    shrink_lruvec
> >    mem_cgroup_iter
> >    shrink_node
> >    do_try_to_free_pages
> >    try_to_free_pages
> >    __alloc_frozen_pages_noprof
> >    alloc_pages_noprof
> >    pte_alloc_one
> >    __pte_alloc
> >    handle_mm_fault
> > 
> > Nothing promises direct reclaim returns in bounded time, and the scan
> > loop in shrink_lruvec() only calls cond_resched(), which is a no-op on
> > PREEMPTION kernels.  Involuntary preemption is not a Tasks-RCU
> > quiescent state, so the reclaiming task never reports one and becomes a
> > holdout.
> 
> I still don't understand why cond_resched() is being treated as involuntary
> preemption but that is orthogonal to this patch.

The history is that cond_resched() was originally intended to be a
preemption point in any otherwise non-preemptible kernel.  Therefore,
because it is a preemption point, it counts as an involuntary context
switch.

							Thanx, Paul

> > Upgrade it to cond_resched_tasks_rcu_qs(), which reports a quiescent
> > state even when cond_resched() does nothing.
> > 
> > PS: This has been discussed in [1]
> > 
> > Link: https://lore.kernel.org/all/amdWVTs0WKOxguxP@gmail.com/ [1]
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Breno Leitao <leitao@debian.org>
> 
> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
>