linux-kernel - Re: [PATCH v2] mm/vmscan: skip increasing kswapd

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Message-Id: <20251025214007.736d659ee266a416c40aa6e5@linux-foundation.org>
Date: Sat, 25 Oct 2025 21:40:07 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Jiayuan Chen <jiayuan.chen@...ux.dev>
Cc: linux-mm@...ck.org, Johannes Weiner <hannes@...xchg.org>, David
 Hildenbrand <david@...hat.com>, Michal Hocko <mhocko@...nel.org>, Qi Zheng
 <zhengqi.arch@...edance.com>, Shakeel Butt <shakeel.butt@...ux.dev>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Axel Rasmussen
 <axelrasmussen@...gle.com>, Yuanchu Xie <yuanchu@...gle.com>, Wei Xu
 <weixugc@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/vmscan: skip increasing kswapd_failures when
 reclaim was boosted

On Fri, 24 Oct 2025 10:27:11 +0800 Jiayuan Chen <jiayuan.chen@...ux.dev> wrote:

> We encountered a scenario where direct memory reclaim was triggered,
> leading to increased system latency:

Who is "we", if I may ask?

> 1. The memory.low values set on host pods are actually quite large, some
>    pods are set to 10GB, others to 20GB, etc.
> 2. Since most pods have memory protection configured, each time kswapd is
>    woken up, if a pod's memory usage hasn't exceeded its own memory.low,
>    its memory won't be reclaimed.
> 3. When applications start up, rapidly consume memory, or experience
>    network traffic bursts, the kernel reaches steal_suitable_fallback(),
>    which sets watermark_boost and subsequently wakes kswapd.
> 4. In the core logic of kswapd thread (balance_pgdat()), when reclaim is
>    triggered by watermark_boost, the maximum priority is 10. Higher
>    priority values mean less aggressive LRU scanning, which can result in
>    no pages being reclaimed during a single scan cycle:
> 
> if (nr_boost_reclaim && sc.priority == DEF_PRIORITY - 2)
>     raise_priority = false;
> 
> 5. This eventually causes pgdat->kswapd_failures to continuously
>    accumulate, exceeding MAX_RECLAIM_RETRIES, and consequently kswapd stops
>    working. At this point, the system's available memory is still
>    significantly above the high watermark — it's inappropriate for kswapd
>    to stop under these conditions.
> 
> The final observable issue is that a brief period of rapid memory
> allocation causes kswapd to stop running, ultimately triggering direct
> reclaim and making the applications unresponsive.
> 

This logic appears to be at least eight years old.  Can you suggest why
this issue is being observed after so much time?

>
> ...
>
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -7128,7 +7128,12 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx)
>  		goto restart;
>  	}
>  
> -	if (!sc.nr_reclaimed)
> +	/*
> +	 * If the reclaim was boosted, we might still be far from the
> +	 * watermark_high at this point. We need to avoid increasing the
> +	 * failure count to prevent the kswapd thread from stopping.
> +	 */
> +	if (!sc.nr_reclaimed && !boosted)
>  		atomic_inc(&pgdat->kswapd_failures);
>  

Thanks, I'll toss it in for testing and shall await reviewer input
before proceeding further.