linux-kernel - Re: [PATCH v1] mm/vmscan: Add retry logic for cgroups with memory.low in kswapd

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Message-ID: <v56ql5ptxyx4x2j7nffznhs3osjghkoj3dlkfbpohotuof3yiv@gf74er4pf34v>
Date: Fri, 7 Nov 2025 16:09:32 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Michal Hocko <mhocko@...e.com>
Cc: Jiayuan Chen <jiayuan.chen@...ux.dev>, linux-mm@...ck.org, 
	Andrew Morton <akpm@...ux-foundation.org>, Axel Rasmussen <axelrasmussen@...gle.com>, 
	Yuanchu Xie <yuanchu@...gle.com>, Wei Xu <weixugc@...gle.com>, 
	Johannes Weiner <hannes@...xchg.org>, David Hildenbrand <david@...hat.com>, 
	Qi Zheng <zhengqi.arch@...edance.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] mm/vmscan: Add retry logic for cgroups with
 memory.low in kswapd

On Fri, Nov 07, 2025 at 02:22:14PM +0100, Michal Hocko wrote:
> Sorry for late reply.
> 
> On Mon 20-10-25 10:11:23, Jiayuan Chen wrote:
> [...]
> > To provide more context about our specific setup:
> > 
> > 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 limits 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 to me sounds like something to be addressed in the watermark
> boosting code. I do not think we should be breaching low limit for that
> (opportunistic) reclaim.

Jiayuan already posted v2 with different approach. We can move the
discussion there.

http://lore.kernel.org/20251024022711.382238-1-jiayuan.chen@linux.dev