Message-ID: <xhsmhbjroade7.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Mon, 19 May 2025 16:22:40 +0200
From: Valentin Schneider <vschneid@...hat.com>
To: Nam Cao <namcao@...utronix.de>, 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>, Ben Segall
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 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 19/05/25 11:45, Nam Cao wrote:
> 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.
>

FWIW I'm thinking the same.

[1] fixes the CFS throttling circular dependency on RT but also improves
tail latency when CFS throttling is in use (so not just for RT).
[3] is specific to epoll but gets rid of the rwsem which is a win overall.

I need to go get my reviewing hat back on and stare at 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

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