linux-kernel - Re: [RT BUG] Stall caused by eventpoll, rwlocks and CFS bandwidth controller

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Message-ID: <20250409135720.YuroItHp@linutronix.de>
Date: Wed, 9 Apr 2025 15:57:20 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Jan Kiszka <jan.kiszka@...mens.com>
Cc: Valentin Schneider <vschneid@...hat.com>,
	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	kprateek.nayak@....com, Thomas Gleixner <tglx@...utronix.de>,
	Juri Lelli <juri.lelli@...hat.com>,
	Clark Williams <williams@...hat.com>,
	"Luis Claudio R. Goncalves" <lgoncalv@...hat.com>,
	Andreas Ziegler <ziegler.andreas@...mens.com>,
	Felix Moessbauer <felix.moessbauer@...mens.com>,
	Florian Bezdeka <florian.bezdeka@...mens.com>
Subject: Re: [RT BUG] Stall caused by eventpoll, rwlocks and CFS bandwidth
 controller

On 2025-04-09 15:52:19 [+0200], Jan Kiszka wrote:
> On second glance, Frederic's patch would probably also avoid the issue
> we are seeing as it should bring PI to the CFS-throttled read-lock
> holder (which is now a write-lock holder).
> 
> But given how old that proposal is, I assume the performance impact was
> even for the RT kernel too much, wasn't it?

I don't remember any numbers and how bad things would get just the fear
of it. But I guess this is workload related in terms how much does the
RW lock improve the situation. RT then will multiple the CPU and reader
resources to the point where it does not scale and present you the
lockups.

> Jan

Sebastian