linux-kernel - [patch 0/3] lib/percpu_counter, cpu/hotplug: Cure the cpu_dying

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Message-ID: <20230414162755.281993820@linutronix.de>
Date:   Fri, 14 Apr 2023 18:30:42 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Valentin Schneider <vschneid@...hat.com>,
        Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        Dave Chinner <dchinner@...hat.com>,
        Yury Norov <yury.norov@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Ye Bin <yebin10@...wei.com>, linux-mm@...ck.org
Subject: [patch 0/3] lib/percpu_counter, cpu/hotplug: Cure the cpu_dying_mask woes

Hi!

The cpu_dying_mask is not only undocumented but also to some extent a
misnomer. It's purpose is to capture the last direction of a cpu_up() or
cpu_down() operation taking eventual rollback operations into account.

cpu_dying mask is not really useful for general consumption. The
cpu_dying_mask bits are sticky even after cpu_up() or cpu_down() completes.

A recent fix to plug a race in the per CPU counter code picked
cpu_dying_mask to cure it. Unfortunately this does not work as the author
probably expected and the behaviour of cpu_dying_mask is not easy to change
without breaking the only other and initial user, the scheduler.

This series addresses this by:

   1) Reworking the per CPU counter hotplug mechanism so the race is fully
      plugged without using cpu_dying_mask

   2) Replacing the cpu_dying_mask logic with hotplug core internal state
      which is exposed to the scheduler with a properly documented
      function.

The series is also available from git:

  git://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git smp/dying_mask

Thanks

	tglx
---
 include/linux/cpuhotplug.h |    2 -
 include/linux/cpumask.h    |   21 ----------------
 kernel/cpu.c               |   45 +++++++++++++++++++++++++++++------
 kernel/sched/core.c        |    4 +--
 kernel/smpboot.h           |    2 +
 lib/percpu_counter.c       |   57 +++++++++++++++++++--------------------------
 6 files changed, 67 insertions(+), 64 deletions(-)