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

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Message-ID: <438a5250-30c0-4820-9084-cdef50f67865@siemens.com>
Date: Tue, 15 Apr 2025 07:35:50 +0200
From: Jan Kiszka <jan.kiszka@...mens.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 K Prateek Nayak <kprateek.nayak@....com>
Cc: Aaron Lu <ziqianlu@...edance.com>,
 Valentin Schneider <vschneid@...hat.com>, linux-rt-users@...r.kernel.org,
 linux-kernel@...r.kernel.org, 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 14.04.25 17:05, Sebastian Andrzej Siewior wrote:
> On 2025-04-14 20:20:04 [+0530], K Prateek Nayak wrote:
>> Note: I could not reproduce the splat with !PREEMPT_RT kernel
>> (CONFIG_PREEMPT=y) or with small loops counts that don't exhaust the
>> cfs bandwidth.
> 
> Not sure what this has to do with anything.
> On !RT the read_lock() in the timer can be acquired even with a pending
> writer. The writer keeps spinning until the main thread is gone. There
> should be no RCU boosting but the RCU still is there, too.
> 
> On RT the read_lock() in the timer block, the write blocks, too. So
> every blocker on the lock is scheduled out until the reader is gone. On
> top of that, the reader gets RCU boosted with FIFO-1 by default to get
> out.

There is no boosting of the active readers on RT as there is no
information recorded about who is currently holding a read lock. This is
the whole point why rwlocks are hairy with RT, I thought.

Jan

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