linux-kernel - Re: Futex hash_bucket lock can break isolation and cause priority inversion on RT

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Message-ID: <ZwY-RZ51-1fD_9mr@jlelli-thinkpadt14gen4.remote.csb>
Date: Wed, 9 Oct 2024 09:26:45 +0100
From: Juri Lelli <juri.lelli@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Darren Hart <dvhart@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	André Almeida <andrealmeid@...lia.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	Valentin Schneider <vschneid@...hat.com>,
	Waiman Long <longman@...hat.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: Futex hash_bucket lock can break isolation and cause priority
 inversion on RT

Hi Peter,

On 08/10/24 19:38, Peter Zijlstra wrote:
> On Tue, Oct 08, 2024 at 04:22:26PM +0100, Juri Lelli wrote:
> > Does this report make any sense? If it does, has this issue ever been
> > reported and possibly discussed? I guess it’s kind of a corner case, but
> > I wonder if anybody has suggestions already on how to possibly try to
> > tackle it from a kernel perspective.
> 
> Any shared lock can cause such havoc. Futex hash buckets is just one of
> a number of very popular ones that's relatively easy to hit.

Ah yes indeed. Just thought that if we have ideas on how to possibly
make this better it might still be worthwhile, even if it won't fix all
issues.

> I do have some futex-numa patches still pending, but they won't
> magically sure this either. Userspace needs help at the very least.

Thanks!
Juri