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Date: Tue, 14 Oct 2025 12:48:39 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Lyude Paul <lyude@redhat.com>
Cc: rust-for-linux@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
	Boqun Feng <boqun.feng@gmail.com>, linux-kernel@vger.kernel.org,
	Daniel Almeida <daniel.almeida@collabora.com>,
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	Uladzislau Rezki <urezki@gmail.com>,
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	=?iso-8859-1?Q?Bj=F6rn?= Roy Baron <bjorn3_gh@protonmail.com>,
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	K Prateek Nayak <kprateek.nayak@amd.com>,
	"open list:CPU FREQUENCY SCALING FRAMEWORK" <linux-pm@vger.kernel.org>
Subject: Re: [PATCH v13 01/17] preempt: Track NMI nesting to separate per-CPU
 counter
Message-ID: <20251014104839.GN4067720@noisy.programming.kicks-ass.net>
References: <20251013155205.2004838-1-lyude@redhat.com>
 <20251013155205.2004838-2-lyude@redhat.com>
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In-Reply-To: <20251013155205.2004838-2-lyude@redhat.com>

On Mon, Oct 13, 2025 at 11:48:03AM -0400, Lyude Paul wrote:

>  #define __nmi_enter()						\
>  	do {							\
>  		lockdep_off();					\
>  		arch_nmi_enter();				\
> -		BUG_ON(in_nmi() == NMI_MASK);			\
> -		__preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET);	\
> +		BUG_ON(__this_cpu_read(nmi_nesting) == UINT_MAX);	\
> +		__this_cpu_inc(nmi_nesting);			\

An NMI that nests from here..

> +		__preempt_count_add(HARDIRQ_OFFSET);		\
> +		if (__this_cpu_read(nmi_nesting) == 1)		\

.. until here, will see nmi_nesting > 1 and not set NMI_OFFSET.

> +			__preempt_count_add(NMI_OFFSET);	\