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

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Message-ID: <Zw6o_OyhzYd6hfjZ@arm.com>
Date: Tue, 15 Oct 2024 18:40:12 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: "Christoph Lameter (Ampere)" <cl@...two.org>
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,
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	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 Tue, Oct 15, 2024 at 10:17:13AM -0700, Christoph Lameter (Ampere) wrote:
> On Tue, 15 Oct 2024, Catalin Marinas wrote:
> > > Setting of need_resched() from another processor involves sending an IPI
> > > after that was set. I dont think we need to smp_cond_load_relaxed since
> > > the IPI will cause an event. For ARM a WFE would be sufficient.
> >
> > I'm not worried about the need_resched() case, even without an IPI it
> > would still work.
> >
> > The loop_count++ side of the condition is supposed to timeout in the
> > absence of a need_resched() event. You can't do an smp_cond_load_*() on
> > a variable that's only updated by the waiting CPU. Nothing guarantees to
> > wake it up to update the variable (the event stream on arm64, yes, but
> > that's generic code).
> 
> Hmm... I have WFET implementation here without smp_cond modelled after
> the delay() implementation ARM64 (but its not generic and there is
> an additional patch required to make this work. Intermediate patch
> attached)

At least one additional patch ;). But yeah, I suggested hiding all this
behind something like smp_cond_load_timeout() which would wait on
current_thread_info()->flags but with a timeout. The arm64
implementation would follow some of the logic in __delay(). Others may
simply poll with cpu_relax().

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.

-- 
Catalin