linux-kernel - Re: [PATCH v2 2/2] memcg: Don't trigger hung task warnings when memcg is releasing resources.

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Message-ID: <20250924063219.GR4067720@noisy.programming.kicks-ass.net>
Date: Wed, 24 Sep 2025 08:32:19 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Julian Sun <sunjunchao@...edance.com>
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, lance.yang@...ux.dev,
	mhiramat@...nel.org, yangyicong@...ilicon.com, will@...nel.org,
	dianders@...omium.org, mingo@...nel.org, lihuafei1@...wei.com,
	hannes@...xchg.org, mhocko@...nel.org, roman.gushchin@...ux.dev,
	shakeel.butt@...ux.dev, muchun.song@...ux.dev, tj@...nel.org
Subject: Re: [PATCH v2 2/2] memcg: Don't trigger hung task warnings when
 memcg is releasing resources.

On Wed, Sep 24, 2025 at 11:41:00AM +0800, Julian Sun wrote:
> Hung task warning in mem_cgroup_css_free() is undesirable and
> unnecessary since the behavior of waiting for a long time is
> expected.
> 
> Use touch_hung_task_detector() to eliminate the possible
> hung task warning.
> 
> Signed-off-by: Julian Sun <sunjunchao@...edance.com>

Still hate this. It is not tied to progress. If progress really stalls,
no warning will be given.

>  mm/memcontrol.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 8dd7fbed5a94..fc73a56372a4 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -63,6 +63,7 @@
>  #include <linux/seq_buf.h>
>  #include <linux/sched/isolation.h>
>  #include <linux/kmemleak.h>
> +#include <linux/nmi.h>
>  #include "internal.h"
>  #include <net/sock.h>
>  #include <net/ip.h>
> @@ -3912,8 +3913,15 @@ static void mem_cgroup_css_free(struct cgroup_subsys_state *css)
>  	int __maybe_unused i;
>  
>  #ifdef CONFIG_CGROUP_WRITEBACK
> -	for (i = 0; i < MEMCG_CGWB_FRN_CNT; i++)
> +	for (i = 0; i < MEMCG_CGWB_FRN_CNT; i++) {
> +		/*
> +		 * We don't want the hung task detector to report warnings
> +		 * here since there's nothing wrong if the writeback work
> +		 * lasts for a long time.
> +		 */
> +		touch_hung_task_detector(current);
>  		wb_wait_for_completion(&memcg->cgwb_frn[i].done);
> +	}
>  #endif
>  	if (cgroup_subsys_on_dfl(memory_cgrp_subsys) && !cgroup_memory_nosocket)
>  		static_branch_dec(&memcg_sockets_enabled_key);
> -- 
> 2.39.5
>