linux-kernel - [RFC PATCH v2 0/4] sched+mm: Track lazy active mm existence with hazard pointers

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Message-Id: <20241004182734.1761555-1-mathieu.desnoyers@efficios.com>
Date: Fri,  4 Oct 2024 14:27:30 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: linux-kernel@...r.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.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>,
	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: [RFC PATCH v2 0/4] sched+mm: Track lazy active mm existence with hazard pointers

[ Based on recent discussion with Linus, it appears that this series
  just proves that it's time to get rid of active mm altogether:

  https://lore.kernel.org/lkml/CAHk-=wgztWbA4z85xKob4eS9P=Nt5h4j=HnN+Pc90expskiCRA@mail.gmail.com/

  I'm posting a v2 fixing minor issues from v1 in case it can be useful
  to try hazard pointers with other use-cases, or for further
  benchmarking of active mm tracking impact.
]

Hazard pointers appear to be a good fit for replacing refcount based lazy
active mm tracking.

Highlight:

will-it-scale context_switch1_threads

nr threads (-t)     speedup
     1                -0.2%
     2                +0.4%
     3                +0.2%
     6                +0.6%
    12                +0.8%
    24                +3%
    48               +12%
    96               +21%
   192               +28%
   384                +4%
   768                -0.6%

This series applies on top of v6.11.1.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Nicholas Piggin <npiggin@...il.com>
Cc: Michael Ellerman <mpe@...erman.id.au>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Will Deacon <will@...nel.org>
Cc: Alan Stern <stern@...land.harvard.edu>
Cc: John Stultz <jstultz@...gle.com>
Cc: Neeraj Upadhyay <Neeraj.Upadhyay@....com>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: Frederic Weisbecker <frederic@...nel.org>
Cc: Joel Fernandes <joel@...lfernandes.org>
Cc: Josh Triplett <josh@...htriplett.org>
Cc: Uladzislau Rezki <urezki@...il.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Lai Jiangshan <jiangshanlai@...il.com>
Cc: Zqiang <qiang.zhang1211@...il.com>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Waiman Long <longman@...hat.com>
Cc: Mark Rutland <mark.rutland@....com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Vlastimil Babka <vbabka@...e.cz>
Cc: maged.michael@...il.com
Cc: Mateusz Guzik <mjguzik@...il.com>
Cc: Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
Cc: rcu@...r.kernel.org
Cc: linux-mm@...ck.org
Cc: lkmm@...ts.linux.dev

Mathieu Desnoyers (4):
  compiler.h: Introduce ptr_eq() to preserve address dependency
  Documentation: RCU: Refer to ptr_eq()
  hp: Implement Hazard Pointers
  sched+mm: Use hazard pointers to track lazy active mm existence

 Documentation/RCU/rcu_dereference.rst |  38 ++++++-
 Documentation/mm/active_mm.rst        |   9 +-
 arch/Kconfig                          |  32 ------
 arch/powerpc/Kconfig                  |   1 -
 arch/powerpc/mm/book3s64/radix_tlb.c  |  23 +---
 include/linux/compiler.h              |  63 ++++++++++
 include/linux/hp.h                    | 158 ++++++++++++++++++++++++++
 include/linux/mm_types.h              |   3 -
 include/linux/sched/mm.h              |  71 +++++-------
 kernel/Makefile                       |   2 +-
 kernel/exit.c                         |   4 +-
 kernel/fork.c                         |  47 ++------
 kernel/hp.c                           |  46 ++++++++
 kernel/sched/sched.h                  |   8 +-
 lib/Kconfig.debug                     |  10 --
 15 files changed, 350 insertions(+), 165 deletions(-)
 create mode 100644 include/linux/hp.h
 create mode 100644 kernel/hp.c

-- 
2.39.2