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

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Message-ID: <ZwY-y4h7LruimB0O@jlelli-thinkpadt14gen4.remote.csb>
Date: Wed, 9 Oct 2024 09:28:59 +0100
From: Juri Lelli <juri.lelli@...hat.com>
To: Waiman Long <llong@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: Futex hash_bucket lock can break isolation and cause priority
 inversion on RT

Hi Waiman,

On 08/10/24 14:30, Waiman Long wrote:
> On 10/8/24 11:22 AM, Juri Lelli wrote:

...

> > Now, of course by making the latency sensitive application tasks use a
> > higher priority than anything on housekeeping CPUs we could avoid the
> > issue, but the fact that an implicit in-kernel link between otherwise
> > unrelated tasks might cause priority inversion is probably not ideal?
> > Thus this email.
> > 
> > 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.
> 
> Just a question. Is the low latency application using PI futex or the normal
> wait-wake futex? We could use separate set of hash buckets for these
> distinct futex types.

AFAIK it uses normal futexes (or a mix at best). Also I believe it
relies on libraries, so somewhat difficult to tell for certain.

Thanks,
Juri