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

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(-)


             reply	other threads:[~2023-04-14 16:30 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-04-14 16:30 Thomas Gleixner [this message]
2023-04-14 16:30 ` [patch 1/3] lib/percpu_counter: Fix CPU hotplug handling Thomas Gleixner
2023-04-15  5:20   ` Dennis Zhou
2023-04-17  2:09   ` Dave Chinner
2023-04-17  8:09     ` Thomas Gleixner
2023-04-14 16:30 ` [patch 2/3] cpu/hotplug: Remove export of cpu_active_mask and cpu_dying_mask Thomas Gleixner
2023-04-14 16:30 ` [patch 3/3] cpu/hotplug: Get rid of cpu_dying_mask Thomas Gleixner
2023-05-03 11:50 ` [patch 0/3] lib/percpu_counter, cpu/hotplug: Cure the cpu_dying_mask woes Valentin Schneider
2023-12-30 22:39 ` Dennis Zhou

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