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

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Message-ID: <20241007070627.GC18071@noisy.programming.kicks-ass.net>
Date: Mon, 7 Oct 2024 09:06:27 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Linus Torvalds <torvalds@...ux-foundation.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, Oct 05, 2024 at 09:56:24AM -0700, Linus Torvalds wrote:
> 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?

Last time I tried it, it worked fine. I just didn't get around to
actually fully thinking it trough and making sure nothing subtle was
broken etc. Pesky details and such..