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

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Date: Fri, 22 Dec 2023 11:52:08 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Chris Li <chrisl@...nel.org>
Cc: 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>
Subject: Re: [PATCH] mm: swap: async free swap slot cache entries

On Thu, 21 Dec 2023 22:25:39 -0800 Chris Li <chrisl@...nel.org> wrote:

> We discovered that 1% swap page fault is 100us+ while 50% of
> the swap fault is under 20us.
> 
> Further investigation show that a large portion of the time
> spent in the free_swap_slots() function for the long tail case.
> 
> The percpu cache of swap slots is freed in a batch of 64 entries
> inside free_swap_slots(). These cache entries are accumulated
> from previous page faults, which may not be related to the current
> process.
> 
> Doing the batch free in the page fault handler causes longer
> tail latencies and penalizes the current process.
> 
> Move free_swap_slots() outside of the swapin page fault handler into an
> async work queue to avoid such long tail latencies.

This will require a larger amount of total work than the current
scheme.  So we're trading that off against better latency.

Why is this a good tradeoff?