linux-kernel - Re: [PATCH v3 1/5] asm-generic: barrier: Add smp_cond_load_relaxed

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Message-Id: <67b6b738-0f1c-4dd4-817d-95f55ec9272b@app.fastmail.com>
Date: Wed, 13 Aug 2025 18:29:37 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Catalin Marinas" <catalin.marinas@....com>,
 "Ankur Arora" <ankur.a.arora@...cle.com>
Cc: linux-kernel@...r.kernel.org, Linux-Arch <linux-arch@...r.kernel.org>,
 linux-arm-kernel@...ts.infradead.org, bpf@...r.kernel.org,
 "Will Deacon" <will@...nel.org>, "Peter Zijlstra" <peterz@...radead.org>,
 "Andrew Morton" <akpm@...ux-foundation.org>,
 "Mark Rutland" <mark.rutland@....com>, harisokn@...zon.com, cl@...two.org,
 "Alexei Starovoitov" <ast@...nel.org>,
 "Kumar Kartikeya Dwivedi" <memxor@...il.com>, zhenglifeng1@...wei.com,
 xueshuai@...ux.alibaba.com, "Joao Martins" <joao.m.martins@...cle.com>,
 "Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
 "Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
 "Rafael J . Wysocki" <rafael@...nel.org>,
 "Daniel Lezcano" <daniel.lezcano@...aro.org>
Subject: Re: [PATCH v3 1/5] asm-generic: barrier: Add smp_cond_load_relaxed_timewait()

On Wed, Aug 13, 2025, at 18:09, Catalin Marinas wrote:
> On Mon, Aug 11, 2025 at 02:15:56PM -0700, Ankur Arora wrote:
>
>> This also gives the WFET a clear end time (though it would still need
>> to be converted to timer cycles) but the WFE path could stay simple
>> by allowing an overshoot instead of falling back to polling.
>
> For arm64, both WFE and WFET would be woken up by the event stream
> (which is enabled on all production systems). The only reason to use
> WFET is if you need smaller granularity than the event stream period
> (100us). In this case, we should probably also add a fallback from WFE
> to a busy loop.

I think there is a reasonable chance that in the future we may want
to turn the event stream off on systems that support WFET, since that
is better for both low-power systems and virtual machines with CPU
overcommit.

    Arnd