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

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Message-ID: <20250512104828.hjVjvmT3@linutronix.de>
Date: Mon, 12 May 2025 12:48:28 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Chen Ridong <chenridong@...weicloud.com>, paulmck@...nel.org,
	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 2025-05-09 13:18:49 [-0700], Andrew Morton wrote:
> 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?

That sounds like a smaller change. Also, do these 1…64 docker container
play signal ping-pong or is there a real workload behind it?

Sebastian