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

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Message-ID: <53bf468b-1616-3915-f5bc-aa29130b672d@gentwo.org>
Date: Thu, 17 Oct 2024 09:56:13 -0700 (PDT)
From: "Christoph Lameter (Ampere)" <cl@...two.org>
To: Catalin Marinas <catalin.marinas@....com>
cc: Ankur Arora <ankur.a.arora@...cle.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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    peterz@...radead.org, arnd@...db.de, lenb@...nel.org, mark.rutland@....com, 
    harisokn@...zon.com, mtosatti@...hat.com, sudeep.holla@....com, 
    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, Catalin Marinas wrote:

> The behaviour above is slightly different from the current poll_idle()
> implementation. The above is more like poll every timeout period rather
> than continuously poll until either the need_resched() condition is true
> _or_ the timeout expired. From Ankur's email, an IPI may not happen so
> we don't have any guarantee that WFET will wake up before the timeout.
> The only way for WFE/WFET to wake up on need_resched() is to use LDXR to
> arm the exclusive monitor. That's what smp_cond_load_relaxed() does.

Sorry no. The IPI will cause the WFE to continue immediately and not wait
till the end of the timeout period.