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

[Paul E. McKenney]

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.
> 
> 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@xxxxxxxxx/ [1]
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>

Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

> ---
>  mm/vmscan.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 26436059ea394..6ac2fde137b89 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -6023,7 +6023,7 @@ static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
>  			}
>  		}
>  
> -		cond_resched();
> +		cond_resched_tasks_rcu_qs();
>  
>  		if (nr_reclaimed < nr_to_reclaim || proportional_reclaim)
>  			continue;
> 
> ---
> base-commit: 6b8c8af514d739d0335f5579b585e02babe8a727
> change-id: 20260810-rcu_task_shrink_lruvec-711112e87de0
> 
> Best regards,
> --  
> Breno Leitao <leitao@xxxxxxxxxx>
>