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

[Paul E. McKenney]

On Tue, Aug 11, 2026 at 10:54:23AM -0700, Shakeel Butt wrote:
> On Tue, Aug 11, 2026 at 10:38:40AM -0700, Paul E. McKenney wrote:
> > 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.
> 
> Thanks for the explanation. Is cond_resched_tasks_rcu_qs() voluntary or
> involuntary context switch?

That one is voluntary in order to take care of Tasks RCU's need for a
voluntary context switch.

Except that it doesn't bother involving the scheduler at all, but
instead just updates the current task's state with respect to Tasks RCU.
Much faster that way.  ;-)

							Thanx, Paul