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

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Message-ID: <2c232dc6-6a13-e34b-bdcc-691c966796d4@gentwo.org>
Date: Tue, 15 Oct 2024 15:28:50 -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, 
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Subject: Re: [PATCH v8 01/11] cpuidle/poll_state: poll via
 smp_cond_load_relaxed()

On Tue, 15 Oct 2024, Ankur Arora wrote:

> > Alternatively, if we get an IPI anyway, we can avoid smp_cond_load() and
> > rely on need_resched() and some new delay/cpu_relax() API that waits for
> > a timeout or an IPI, whichever comes first. E.g. cpu_relax_timeout()
> > which on arm64 it's just a simplified version of __delay() without the
> > 'while' loops.
>
> AFAICT when polling (which we are since poll_idle() calls
> current_set_polling_and_test()), the scheduler will elide the IPI
> by remotely setting the need-resched bit via set_nr_if_polling().

The scheduler runs on multiple cores. The core on which we are
running this code puts the core into a wait state so the scheduler does
not run on this core at all during the wait period.

The other cores may run scheduler functions and set the need_resched bit
for the core where we are currently waiting.

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.

> Once we stop polling then the scheduler should take the IPI path
> because call_function_single_prep_ipi() will fail.

The IPI stops the polling. IPI is an interrupt.