linux-kernel - Re: [PATCH 17/19] srcu: Optimize SRCU-fast-updown for arm64

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Message-ID: <98fa1a01-400b-4496-997c-1f84f33dd1ad@efficios.com>
Date: Mon, 3 Nov 2025 14:22:00 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: paulmck@...nel.org
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
 rostedt@...dmis.org, Catalin Marinas <catalin.marinas@....com>,
 Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 linux-arm-kernel@...ts.infradead.org, bpf@...r.kernel.org
Subject: Re: [PATCH 17/19] srcu: Optimize SRCU-fast-updown for arm64

On 2025-11-03 14:17, Paul E. McKenney wrote:
[...]
>> This is effectively a use of split-counters, but the split
>> is across concurrency handling mechanisms rather than across
>> CPUs.
> 
> Ah, got it, thank you!  But we would need an additional softirq counter,
> correct?

Fundamentally it depends on how you want to split frequent vs infrequent
accesses.

If the fast-paths you care about are all in thread context, then you
only need to split between percpu ops vs atomic counters. The per-thread
accesses would use percpu ops, and all the rest use atomics. The "all
the rest" can cover everything else including softirqs, irq, and nmis.

> 
> I will keep this in my back pocket in case Catalin's and Yicong's prefetch
> trick turns out to be problematic, and again, thank you!

You're welcome! :)

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com