linux-kernel - Re: [PATCH v8 01/11] cpuidle/poll_state: poll via smp_cond_load

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Message-ID: <a07fb08b-d9d0-c9cc-8e03-3857d0adffdf@gentwo.org>
Date: Thu, 17 Oct 2024 09:54:34 -0700 (PDT)
From: "Christoph Lameter (Ampere)" <cl@...two.org>
To: Ankur Arora <ankur.a.arora@...cle.com>
cc: Catalin Marinas <catalin.marinas@....com>, linux-pm@...r.kernel.org, 
    kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
    linux-kernel@...r.kernel.org, will@...nel.org, tglx@...utronix.de, 
    mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, 
    x86@...nel.org, hpa@...or.com, pbonzini@...hat.com, wanpengli@...cent.com, 
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    misono.tomohiro@...itsu.com, maobibo@...ngson.cn, 
    joao.m.martins@...cle.com, boris.ostrovsky@...cle.com, 
    konrad.wilk@...cle.com
Subject: Re: [PATCH v8 01/11] cpuidle/poll_state: poll via
 smp_cond_load_relaxed()

On Wed, 16 Oct 2024, Ankur Arora wrote:

> > The other core will wake our core up by sending an IPI. The IPI will
> > invoke a scheduler function on our core and the WFE will continue.
>
> Why? The target core is not sleeping. It is *polling* on a memory
> address (on arm64, via LDXR; WFE). Ergo an IPI is not needed to tell
> it that a need-resched bit is set.

The IPI is sent to interrupt the process that is not sleeping. This is
done so the busy processor can reschedule the currently running process
and respond to the event.

It does not matter if the core is "sleeping" or not.