linux-kernel - Re: [PATCH] mm: swap: async free swap slot cache entries

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Date: Sat, 23 Dec 2023 08:51:58 -0800
From: Chris Li <chrisl@...nel.org>
To: David Rientjes <rientjes@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, Wei Xu <weixugc@...gle.com>, Yu Zhao <yuzhao@...gle.com>, 
	Greg Thelen <gthelen@...gle.com>, Chun-Tse Shao <ctshao@...gle.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, Yosry Ahmed <yosryahmed@...gle.com>, 
	Brain Geffon <bgeffon@...gle.com>, Minchan Kim <minchan@...nel.org>, Michal Hocko <mhocko@...e.com>, 
	Mel Gorman <mgorman@...hsingularity.net>, Huang Ying <ying.huang@...el.com>, 
	Nhat Pham <nphamcs@...il.com>, Johannes Weiner <hannes@...xchg.org>, Kairui Song <kasong@...cent.com>, 
	Zhongkun He <hezhongkun.hzk@...edance.com>, Kemeng Shi <shikemeng@...weicloud.com>, 
	Barry Song <v-songbaohua@...o.com>, Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH] mm: swap: async free swap slot cache entries

Hi David,

On Fri, Dec 22, 2023 at 10:11 PM David Rientjes <rientjes@...gle.com> wrote:
>
> How do you quantify the impact of the delayed swap_entry_free()?
>
> Since the free and memcg uncharge are now delayed, is there not the
> possibility that we stay under memory pressure for longer?  (Assuming at
> least some users are swapping because of memory pressure.)
>
> I would assume that since the free and uncharge itself is delayed that in
> the pathological case we'd actually be swapping *more* until the async
> worker can run.

Thanks for raising this interesting question.

First of all, the swap_entry_free() does not impact "memory.current".
It reduces "memory.swap.current". Technically it is the swap pressure
not memory pressure that suffers the extra delay.

Secondly, we are talking about delaying up to 64 swap entries for a
few microseconds. Where the swap slot cache itself delays the freeing
of the entries for an arbitrary amount of time. It is not freed until
the cache is full of 64 entries. This delay can be seconds or even
minutes. Adding a few microseconds of  extra delay to existing seconds
delay really makes no difference from the swap pressure point of view.

Chris