linux-kernel - Re: [PATCH] eventpoll: Fix priority inversion problem

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Message-ID: <20250519094543.m4bNJP6X@linutronix.de>
Date: Mon, 19 May 2025 11:45:43 +0200
From: Nam Cao <namcao@...utronix.de>
To: Florian Bezdeka <florian.bezdeka@...mens.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	John Ogness <john.ogness@...utronix.de>,
	Clark Williams <clrkwllms@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
	linux-rt-users@...r.kernel.org, Joe Damato <jdamato@...tly.com>,
	Martin Karsten <mkarsten@...terloo.ca>,
	Jens Axboe <axboe@...nel.dk>,
	Frederic Weisbecker <frederic@...nel.org>,
	Valentin Schneider <vschneid@...hat.com>,
	Ben Segall <bsegall@...gle.com>,
	K Prateek Nayak <kprateek.nayak@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Josh Don <joshdon@...gle.com>, Ingo Molnar <mingo@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Xi Wang <xii@...gle.com>, Juri Lelli <juri.lelli@...hat.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Mel Gorman <mgorman@...e.de>,
	Chengming Zhou <chengming.zhou@...ux.dev>,
	Chuyi Zhou <zhouchuyi@...edance.com>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Andreas Ziegler <ziegler.andreas@...mens.com>
Subject: Re: [PATCH] eventpoll: Fix priority inversion problem

On Mon, May 19, 2025 at 11:25:51AM +0200, Florian Bezdeka wrote:
> Hi all,
> 
> sorry for top-posting, but I think it makes sense in this case as I'm
> trying to merge different workstreams, likely working on the same
> problem showing up in different colors.
> 
> Main goal is to make everybody aware of the other stream / patch
> series.
> 
> We have colleagues from Bytedance working on non-RT performance
> optimizations related to CONFIG_CFS_BANDWIDTH at [1]. 
> 
> This series came to light while searching for a solution for a RT
> lockup, reported at [2]. 
> 
> We heavily tested [1] during the last month on RT and can report
> success now. In our tests we saw read-lock holder preemption only
> within the epoll interface. It might be that [1] fixes more potential
> issues in this regard.
> 
> Today [3] (= the patch I'm replying to, see below) got posted.
> Linutronix reworking the epoll infrastructure.
> 
> I would love to learn how/if the combination, basically [1] and [3] fit
> together.

[1] fixes stall problem involving rw semaphore which epoll uses, but it
doesn't fix the possible priority inversion with epoll

[3] fixes priority inversion problem with epoll by stop using rw semaphore,
but it doesn't do anything about rw semaphore

So I propose we keep both.

Best regards,
Nam

> My understanding right now is, that [1] fixes a CFS issue, throttling
> while holding a lock is not ideal on !RT - but might cause a critical
> lockup on RT - while [3] is addressing a similar (RT) problem in epoll.
> 
> Best regards,
> Florian
> 
> [1] https://lore.kernel.org/all/20250409120746.635476-1-ziqianlu@bytedance.com/
> [2] https://lore.kernel.org/linux-rt-users/xhsmhttqvnall.mognet@vschneid.remote.csb/
> [3] https://lore.kernel.org/linux-rt-users/20250519074016.3337326-1-namcao@linutronix.de/T/#u