linux-kernel - Re: [RFC PATCH 0/4] sched+mm: Track lazy active mm existence with hazard pointers

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Message-ID: <CAHk-=whvxq3378vOn++=ZiOQc9=4N-3ejUWr+dXEJ5ti43kT6w@mail.gmail.com>
Date: Sat, 5 Oct 2024 09:56:24 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, 
	Nicholas Piggin <npiggin@...il.com>, Michael Ellerman <mpe@...erman.id.au>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, "Paul E. McKenney" <paulmck@...nel.org>, 
	Will Deacon <will@...nel.org>, Boqun Feng <boqun.feng@...il.com>, 
	Alan Stern <stern@...land.harvard.edu>, John Stultz <jstultz@...gle.com>, 
	Neeraj Upadhyay <Neeraj.Upadhyay@....com>, Frederic Weisbecker <frederic@...nel.org>, 
	Joel Fernandes <joel@...lfernandes.org>, Josh Triplett <josh@...htriplett.org>, 
	Uladzislau Rezki <urezki@...il.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Lai Jiangshan <jiangshanlai@...il.com>, Zqiang <qiang.zhang1211@...il.com>, 
	Ingo Molnar <mingo@...hat.com>, Waiman Long <longman@...hat.com>, 
	Mark Rutland <mark.rutland@....com>, Thomas Gleixner <tglx@...utronix.de>, 
	Vlastimil Babka <vbabka@...e.cz>, maged.michael@...il.com, Mateusz Guzik <mjguzik@...il.com>, 
	Jonas Oberhauser <jonas.oberhauser@...weicloud.com>, rcu@...r.kernel.org, linux-mm@...ck.org, 
	lkmm@...ts.linux.dev
Subject: Re: [RFC PATCH 0/4] sched+mm: Track lazy active mm existence with
 hazard pointers

On Sat, 5 Oct 2024 at 09:16, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Wed, Oct 02, 2024 at 10:39:15AM -0700, Linus Torvalds wrote:
> > So I think the real issue is that "active_mm" is an old hack from a
> > bygone era when we didn't have the (much more involved) full TLB
> > tracking.
>
> I still seem to have these patches that neither Andy nor I ever managed
> to find time to finish:
>
>   https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/log/?h=x86/lazy

Yes, that looks very much like what I had in mind.

In fact, it looks a lot smaller and simpler than what my mental model was.

I was thinking I'd do it by removing "active_mm" entirely from 'struct
task_struct', and turn it into a per-cpu variable instead, and then
try to massage that into some global new world order. That patch
series you point to seems to be much simpler and clearer.

Of course, you also say "never managed to finish", so presumably
there's something completely broken in that series, and it doesn't
actually work?

                   Linus