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Reply-To: Sean Christopherson <seanjc@google.com>
Date: Tue, 12 Mar 2024 12:39:09 -0700
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Message-ID: <20240312193911.1796717-1-seanjc@google.com>
Subject: [PATCH v2 0/2] sched/core: Fix spinlocks vs. PREEMPT_DYNAMIC=y
From: Sean Christopherson <seanjc@google.com>
To: Jonathan Corbet <corbet@lwn.net>, Ingo Molnar <mingo@redhat.com>, 
	Peter Zijlstra <peterz@infradead.org>, Juri Lelli <juri.lelli@redhat.com>, 
	Vincent Guittot <vincent.guittot@linaro.org>, Will Deacon <will@kernel.org>
Cc: linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, 
	Sean Christopherson <seanjc@google.com>, Valentin Schneider <valentin.schneider@arm.com>, 
	Marco Elver <elver@google.com>, Frederic Weisbecker <frederic@kernel.org>, 
	David Matlack <dmatlack@google.com>, Friedrich Weber <f.weber@proxmox.com>, 
	Ankur Arora <ankur.a.arora@oracle.com>, Thomas Gleixner <tglx@linutronix.de>
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Fix a bug in dynamic preemption where the kernel will yield contended
spinlocks (and rwlocks) even if the selected preemption model is "none" or
"voluntary".  I say "bug" because this divergence from PREEMPT_DYNAMIC=n
behavior effectively broke existing KVM configurations, e.g. vCPUs would
get stuck and become unresponsive for multiple seconds if there was heavy
KSM or NUMA balancing activity in the host.

This isn't super urgent, as 6.8 has a fix in KVM for the over-aggressive
yielding (commit d02c357e5bfa ("KVM: x86/mmu: Retry fault before acquiring
mmu_lock if mapping is changing"), but I wouldn't be surprised if the
behavior is causing other performance issues/regressions that are less
severe and/or less visible.

v2:
 - Rebase onto Linus' tree to deal with the code movement to spinlock.h.
 - Opportunistically document the behavior.
 - Add the PREEMPT_AUTO folks to Cc to get their eyeballs/input.

v1: https://lore.kernel.org/all/20240110214723.695930-1-seanjc@google.com

Sean Christopherson (2):
  sched/core: Move preempt_model_*() helpers from sched.h to preempt.h
  sched/core: Drop spinlocks on contention iff kernel is preemptible

 .../admin-guide/kernel-parameters.txt         |  4 +-
 include/linux/preempt.h                       | 41 +++++++++++++++++++
 include/linux/sched.h                         | 41 -------------------
 include/linux/spinlock.h                      | 14 +++----
 4 files changed, 50 insertions(+), 50 deletions(-)


base-commit: b29f377119f68b942369a9366bdcb1fec82b2cda
-- 
2.44.0.278.ge034bb2e1d-goog