linux-kernel - Re: [PATCH v15 3/6] locking/qspinlock: Introduce CNA into the slow path of qspinlock

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Message-ID: <20230804182312.GO212435@hirez.programming.kicks-ass.net>
Date:   Fri, 4 Aug 2023 20:23:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Guo Ren <guoren@...nel.org>
Cc:     Alex Kogan <alex.kogan@...cle.com>, linux@...linux.org.uk,
        mingo@...hat.com, will.deacon@....com, arnd@...db.de,
        longman@...hat.com, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        guohanjun@...wei.com, jglauber@...vell.com,
        steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        dave.dice@...cle.com
Subject: Re: [PATCH v15 3/6] locking/qspinlock: Introduce CNA into the slow
 path of qspinlock

On Fri, Aug 04, 2023 at 10:17:35AM -0400, Guo Ren wrote:

> > See, this is where the ARM64 WFE would come in handy; I don't suppose
> > RISC-V has anything like that?
> Em... arm64 smp_cond_load only could save power consumption or release
> the pipeline resources of an SMT processor. When (Node1 cpu64) is in
> the WFE state, it still needs (Node0 cpu1) to write the value to give
> a cross-NUMA signal. So I didn't see what WFE related to reducing
> cross-Numa transactions, or I missed something. Sorry

The benefit is that WFE significantly reduces the memory traffic. Since
it 'suspends' the core and waits for a write-notification instead of
busy polling the memory location you get a ton less loads.