linux-kernel - Re: [RFC PATCH 48/86] rcu: handle quiescent states for PREEMPT

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Message-ID: <20231121203049.GN8262@noisy.programming.kicks-ass.net>
Date:   Tue, 21 Nov 2023 21:30:49 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org, tglx@...utronix.de,
        torvalds@...ux-foundation.org, linux-mm@...ck.org, x86@...nel.org,
        akpm@...ux-foundation.org, luto@...nel.org, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        willy@...radead.org, mgorman@...e.de, jon.grimm@....com,
        bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        jgross@...e.com, andrew.cooper3@...rix.com, mingo@...nel.org,
        bristot@...nel.org, mathieu.desnoyers@...icios.com,
        geert@...ux-m68k.org, glaubitz@...sik.fu-berlin.de,
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        krypton@...ich-teichert.org, rostedt@...dmis.org,
        David.Laight@...lab.com, richard@....at, mjguzik@...il.com
Subject: Re: [RFC PATCH 48/86] rcu: handle quiescent states for PREEMPT_RCU=n

On Tue, Nov 21, 2023 at 11:25:18AM -0800, Paul E. McKenney wrote:
> #define preempt_enable() \
> do { \
> 	barrier(); \
> 	if (!IS_ENABLED(CONFIG_PREEMPT_RCU) && raw_cpu_read(rcu_data.rcu_urgent_qs) && \
> 	    (preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK | HARDIRQ_MASK | NMI_MASK) == PREEMPT_OFFSET) &&
> 	    !irqs_disabled()) \
> 		rcu_all_qs(); \
> 	if (unlikely(preempt_count_dec_and_test())) { \
> 		__preempt_schedule(); \
> 	} \
> } while (0)

Aaaaahhh, please no. We spend so much time reducing preempt_enable() to
the minimal thing it is today, this will make it blow up into something
giant again.