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

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Message-ID: <be4fa21d-0b96-45de-ac5e-bdb54bee335e@siemens.com>
Date: Wed, 9 Apr 2025 15:41:50 +0200
From: Jan Kiszka <jan.kiszka@...mens.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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 09.04.25 15:21, Sebastian Andrzej Siewior wrote:
> 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…
> 

We know that epoll is not a good idea for RT programs. However, our
problem is that already non-RT programs manage to lock up an RT-enabled
system.

We are currently collecting more data to show what we are seeing, plus
will try out the latest patches on the latest kernels.

Jan

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