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

[Shakeel Butt <shakeel.butt@linux.dev>]

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.

> 
> 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>