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Date: Wed, 24 Apr 2024 13:08:25 -0700
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Subject: Re: [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>, linux-doc@vger.kernel.org, 
	linux-kernel@vger.kernel.org, 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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On Tue, Mar 12, 2024, Sean Christopherson wrote:
> 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.

Anyone have any thoughts on how to move this forward?  I have a hard time
believing no one has opinions on this code :-)

> 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
>