linux-kernel - Re: [RESEND][PATCH v21 0/6] Donor Migration for Proxy Execution (v21)

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Message-ID: <CANDhNCqx1cGRH=H5ze5LWdjTzA40Ajf5pmjrxj==oh_FcTd2oA@mail.gmail.com>
Date: Thu, 11 Sep 2025 16:21:26 -0700
From: John Stultz <jstultz@...gle.com>
To: Juri Lelli <juri.lelli@...hat.com>
Cc: LKML <linux-kernel@...r.kernel.org>, Joel Fernandes <joelagnelf@...dia.com>, 
	Qais Yousef <qyousef@...alina.io>, Ingo Molnar <mingo@...hat.com>, 
	Peter Zijlstra <peterz@...radead.org>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Valentin Schneider <vschneid@...hat.com>, 
	Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, 
	Zimuzo Ezeozue <zezeozue@...gle.com>, Mel Gorman <mgorman@...e.de>, Will Deacon <will@...nel.org>, 
	Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>, 
	"Paul E. McKenney" <paulmck@...nel.org>, Metin Kaya <Metin.Kaya@....com>, 
	Xuewen Yan <xuewen.yan94@...il.com>, K Prateek Nayak <kprateek.nayak@....com>, 
	Thomas Gleixner <tglx@...utronix.de>, Daniel Lezcano <daniel.lezcano@...aro.org>, 
	Suleiman Souhlal <suleiman@...gle.com>, kuyo chang <kuyo.chang@...iatek.com>, hupu <hupu.gm@...il.com>, 
	kernel-team@...roid.com, Luca Abeni <luca.abeni@...tannapisa.it>, 
	Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>, yurand2000@...il.com
Subject: Re: [RESEND][PATCH v21 0/6] Donor Migration for Proxy Execution (v21)

On Thu, Sep 11, 2025 at 6:59 AM Juri Lelli <juri.lelli@...hat.com> wrote:
> On 04/09/25 00:21, John Stultz wrote:
> > I’d really appreciate any feedback or review thoughts on the
> > full series as well.
>
> I current have the following on top of your complete series
>
> https://github.com/jlelli/linux/commits/experimental/eval-mbwi/
> https://github.com/jlelli/linux experimental/eval-mbwi
>
> of which
>
> https://github.com/jlelli/linux/commit/9d4bbb1aca624e76e5b34938d848dc9a418c6146
>
> introduces the testing (M-BWI is Multiprocessor Bandwidth Inheritance)
> infra and the rest some additional tracepoints (based on Gabriele's
> patch) to get more DEADLINE info out of tests (in conjuction with
> sched_tp [1]).
>
> Nothing bit to report just yet, mainly spent time getting this working.

Very cool to see! I'll have to pull those and take a look at it!

And I'm of course very interested to hear if you find anything with
the proxy set that I need to revise.

> One thing I noticed thouh (and probably forgot from previous
> discussions) is that spin_on_owner might be 'confusing' from an
> RT/DEADLINE perspective as it deviates from what one expects from the
> ideal theoretical world (as tasks don't immediately block and
> potentially donate). Not sure what to do about it. Maybe special case it
> for RT/DEADLINE, but just started playing with it.

Can you refresh me a bit on why blocking to donate is preferred? If
the lock owner is running, there's not much that blocking to donate
would help with. Does this concern not apply to the current mutex
logic without proxy?  With proxy-exec, I'm trying to preserve the
existing mutex behavior of spin_on_owner, with the main tweak is just
the lock handoff to the current donor when we are proxying, otherwise
the intent is it should be the same.

Now, I do recognize that rt_mutexes and mutexes do have different lock
handoff requirements for RT tasks (needs to strictly go to the highest
priority waiter, and we can't let a lower priority task steal it),
which is why I've not yet enabled proxy-exec on rt_mutexes.

> Anyway, I will keep playing with all this. Just wanted to give
> you/others a quick update. Also adding Luca, Tommaso and Yuri to the
> thread so that they are aware of the testing framework. :)

Thanks so much for sharing!
-john