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

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Message-ID: <20250409132149.rFrzBCrP@linutronix.de>
Date: Wed, 9 Apr 2025 15:21:49 +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 08:41:44 [+0200], Jan Kiszka wrote:
> We are hunting for quite some time sporadic lock-ups or RT systems, 
> first only in the field (sigh), now finally also in the lab. Those have 
> a fairly high overlap with what was described here. Our baselines so 
> far: 6.1-rt, Debian and vanilla. We are currently preparing experiments 
> with latest mainline.
> 
> While this thread remained silent afterwards, we have found [1][2][3] as 
> apparently related. But this means we are still with this RT bug, even 
> in latest 6.15-rc1?

Not sure the commits are related. The problem here is that RW locks are
not really real time friendly. Frederick had a simple fix to it
	https://lore.kernel.org/all/20210825132754.GA895675@lothringen/

but yeah. The alternative, which I didn't look into, would be to replace
the reader side with RCU so we would just have the writer lock. That
mean we need to RW lock because of performance…

> Jan
> 
> [1] https://lore.kernel.org/lkml/20231030145104.4107573-1-vschneid@redhat.com/
> [2] https://lore.kernel.org/lkml/20240202080920.3337862-1-vschneid@redhat.com/
> [3] https://lore.kernel.org/lkml/20250220093257.9380-1-kprateek.nayak@amd.com/
> 

Sebastian