linux-kernel - Re: [RFC next v2 0/5] ucount: add rlimit cache for ucount

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Message-Id: <20250509131849.112545d60dd7bb2d28c3b966@linux-foundation.org>
Date: Fri, 9 May 2025 13:18:49 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Chen Ridong <chenridong@...weicloud.com>
Cc: paulmck@...nel.org, bigeasy@...utronix.de, legion@...nel.org,
 roman.gushchin@...ux.dev, brauner@...nel.org, tglx@...utronix.de,
 frederic@...nel.org, peterz@...radead.org, oleg@...hat.com,
 joel.granados@...nel.org, viro@...iv.linux.org.uk,
 lorenzo.stoakes@...cle.com, avagin@...gle.com, mengensun@...cent.com,
 linux@...ssschuh.net, jlayton@...nel.org, ruanjinjie@...wei.com,
 kees@...nel.org, linux-kernel@...r.kernel.org, lujialin4@...wei.com
Subject: Re: [RFC next v2 0/5] ucount: add rlimit cache for ucount

On Fri,  9 May 2025 07:20:49 +0000 Chen Ridong <chenridong@...weicloud.com> wrote:

> The will-it-scale test case signal1 [1] has been observed. and the test
> results reveal that the signal sending system call lacks linearity.
> To further investigate this issue, we initiated a series of tests by
> launching varying numbers of dockers and closely monitored the throughput
> of each individual docker. The detailed test outcomes are presented as
> follows:
> 
> 	| Dockers     |1      |4      |8      |16     |32     |64     |
> 	| Throughput  |380068 |353204 |308948 |306453 |180659 |129152 |
> 
> The data clearly demonstrates a discernible trend: as the quantity of
> dockers increases, the throughput per container progressively declines.
> In-depth analysis has identified the root cause of this performance
> degradation. The ucouts module conducts statistics on rlimit, which
> involves a significant number of atomic operations. These atomic
> operations, when acting on the same variable, trigger a substantial number
> of cache misses or remote accesses, ultimately resulting in a drop in
> performance.

Did you consider simply turning that atomic_t counter into a
percpu_counter?